Why Did Horus Betray the Emperor? | Warhammer 40k Lore 

Male Pattern Baldness..... Shave le Balls : manscaped.com/majorkill Tickle le Balls : majorkill.com/

It’s because the emperor kept the stc containing the information on how to do a hair transplant secret and Horus wasn’t too happy about that

I've always thought it was strange that Magnus could shatter the unshatterable wards around the Palace but could barely get through to Horus.

During the vision on Davin, Horus made it clear that he didn't like Magnus or Erebus, but wanted to forge his own path. What confused me was why Horus was such a dickhead immediately? After his war with the Interex, where he was very regretful of how things went down, he became more and more egotistical. Dan Abnett wrote Horus as a very humble primarch, yet Graham McNeil wrote Horus as arrogant even before his wounding on Davin. It would have been better if McNeil and Abnett talked more while writing the books, so that Horus in False Gods would be consistent with Horus from Horus Rising at least initially. Also, if Horus's fall was a lot more gradual, it would make it a lot more believable for more of the legion to take subtle steps to damnation. Sure, there are always the hotheads like Abaddon who was eager for Horus to go to war with the Auretian Technocracy, but Horus accepting the council of Erebus so eagerly and changing so rapidly was very jarring.

I'm surprised the writers never go into more detail on something so crucial.

From the point of view of Horus, he actually went to the past, to the moment when the primarchs were scattered to the warp. In the book it describes how the emperor recognized Horus when he told him who he was and seemed to understand and accept Horus' fall as a part of his plan, that is why Horus betrayed him, because he saw (or thought he saw) the real face of the emperor, an uncaring despot willing to sacrifice him for his own goals, which cemented the future Herebus had shown him as already decided

I always thought (and still do think) that most of the primarchs went traitor really fucking easily. The only 2 I really give it to are Angron (even if I still think he sucks) and Magnus (even though his fall was more chaos and his brothers fucking with him). Everyone else fell like a card castle lol. Horus especially turned faster than a revolving door it took one dude going "hey dude emp bad" and he was like "shit you know what u right"

I think when Horus will get his own primarch novel we might get to see why he truly did what he did 👍

repeat after me, what do we blame Horus' fall on? The Warp? No Nurgle? No Lorgar? No Erebus? No The Emperor? No Magnus? No Abaddon and the Mournival for not keeping a closer eye on him, and giving him what he needed not what he wanted? No *TAXES* ? *_Y E S_*

Felt the same way when I read the books. It felt wierd that he would fall the way he did. Fulgrims fall for example was much better written and actually understandable. Oh well, the script needed it to happen for the plot to move forward I guess.

He could have been the best of us... I think that makes his fall all the more sad.

I think that while he did recover, the warp power Erebus used to "heal" him did a number on his soul. So despite knowing Erebus is a lying asshole, his negative side started to show. Doubting the Emperor, fearing for his fate ending like the Thunder Legion, and the others stated in the vid, he was slowly becoming vulnerable to the temptations of Chaos.

Did Konrad actually fall to Chaos or just to madness? Fulgrim should have realised something was wrong but he was written like a nincompoop. Mortarion got tricked by his best friend. Perturabo had levels of bitterness not seen anywhere else in the cosmos. Alpharius/Omegon believed some weird aliens. Horus didn’t want to die. Angron/Lorgar doing it for the lols and Jesus or something.

I believe that since the Astartes were temporary ( like the thunder warriors) , the Big E had planned for the Astartes to do as they did so they can wipe each other out. He only needed them for the crusade like he only needed the thunder warriors for the Unification wars. I just think it went a little sideways and became something the Emperor didnt foresee.

In the Slave to Darkness novel you understand that Horus didn"t betray the Emperor, he just die in Davin, his soul split between the four gods and his body no more really controlled by him but by chaos

Just noticed after all this damn time that the Emperor, the primarchs and them going traitor for various reasons is inspired by Arthurian legends.

The story was hastened, from the first two novells - there wasn't enough argumentation for his betrayal, if the authors had invested more time on the primarchs (Horus) than secondary characters and over excessive descriptive "word-pooping" they could have fleshed out his story better.

I don’t know why but every time you describe a primarch all I can imagine is playing dynasties warriors set in the 40k setting! Thanks for the great descriptions 😉

I literally died at the “three short stories narrated by Toby Longworth” line. Keep up the good work, pal!

Don't forget about how he felt about his brothers that were were removed that helped in the betrayal.

Honestly i think learning the fate of the thunder warriors should have been the key thing that caused horus’ fall, that is something that would be earth shattering for him and something erebus could easily use to corrupt him. It would be easily for him to believe that the primarchs and their legions would be purged as obsolete relics at the end of the crusade to make way for the emperors vision because it already had precedent with the thunder warriors.

I just read the book and it seems so sudden and a complete 180 on his personality from a very humble human person to evil and genocidal almost instantly in the book

The Emperor: Emanates a literal golden halo. Also the Emperor: Lmao, why would you think I’m a god??

Can you fuking make a video about alchoul in warhemer 40k

The reason why Horus betrayed the emperor was because of the emperor. He realized what he was, what he was made for, and had a crisis of spirit. He wanted to be more then what he was designed to do. So he turned to chaos. Also the fact of his ignorance of chaos made it easier for him to turn. The emperor erasing parts of his memory was another bad too. Theres alot of clues to horus fall spread out over decades. The emperor is to blame most of all though.

How is this not a 1 second long video where he just screams "NO!"? (based on the text in the thumbnail) Or a 3 second video of "Because he was a whiny bitch." (based on the video title)

Can you make a video about war tactis and strategy in warhemer40k

There is a show called Norsemen on which the bad guy used to have a loving wife and a dear old friend he loved like a brother. Until one day, his friend made laugh of him because he was getting bald and losing the mane he was so proud of, which made everyone at a party he was attending laugh. Of course, he took it in stride and simply walked away from the party, not before starting a fire and locking everyone inside the burning building, included his wife. I am not joking, that show or at least that bit, is a must watch for every Majorkill fan. In my headcanon, Horus fell the exact same way because Magnus told him he was getting bald after interrupting Erebus´ vision.

It seems obvious to me that the magic that infected him had a more powerful effect than just a wound to his body. A wound to his mind that festered and caused rot to set in, changing him entirely.

I am at 6:50 and am just now realizing you've been saying "CHAOS knife" - and not "Cows knife" and "Cows magic"

he betrayed because of hid baldness everyone compared him to caillou

I question why his fall was so quick, these are Arstates and Primarchs they live for centuries yet in 2-3 years after being anointed Warmaster, Horus goes evil despite 200 years off being loyal son, the influence Erebus had seemed too immediate if it spanned a decade or so it would make a bit more sense as the imperium gradually became more and more corrupt as the pressure of Warmaster felt more and more like a burden and then he gets the vision, of seeing the emperor worshipped as a god

If your dad had luscious locks like that and you ended up with a chrome dome, you'd probably be resentful as well

Very well summarized. I have to agree, his reasons for betrayal weren't convincing. I think the main error in the writing is the planned timeline for the novels. The author had a very short amount of time to get Horus from loyal son to Traitor Warmaster. They probably should have shown us many more scenarios where Horus loses his faith and decides to rebel. None of it seemed enough of a reason to start such a monumental war.

Does it have anything to do with Horus discovering the truth behind the making of the Primarchs that they are each a warp god. And perhaps by cutting off ties to the warp with the webway project they would lose their power as well? I genuinely don’t know

I just started my journey into the grimdark and Toby Longworth is the voice of Warhammer to me. I myself am bald, but I'm not insecure about it. Still, I want Toby to read us the tales of Horus' insecurity.

Horus, despite always being touted as the best of the primarchs, has never been treated well in the lore. There isn’t enough pre betrayal Horus showing al of his good qualities, and during the late stages of the heresy/siege of terra the writers already start throwing the character under the buss to shill for Abbadon for some reason. Warmaster and Dark Compliance are some of the best pieces of work to enjoy one of the most iconic primarchs

He fought the Emperor cuz he was jealous the Big E had magnificent hair.

My most paletteable head cannon I fall somewhere into Horus being more tragically driven to madness and heresy. Like a part of him broke and was never fixed, for the whole heresy part of him was screaming at his atrocities but incapable of stopping. So he just chose to be Chaos' hand puppet because it was the only way out. Not necessarily true but makes as much sense as everything else.

You said Horus was under pressure as warmaster with the demands from everyone. If he became emperor, being warmaster would have looked like cake.

Did majorkill just quote highboi with "depresso expresso" or the reverse i cant remember but my point still stands. 😆

Ah yes. The chaos knife, "the Anthanamane"

FYI, the blade heller that corrupted Horus was a sword, not a knife. The knives where shards taken from said sword.

probably wanted to protect sanguinius(among others) from the emperor who he believed would surely kill him and his entire legion plus the families of those in it.

I think Abnett and the other involved writers portrayed a reasonable downfall for the Warmaster, but the significance of many events is so small, they probably aren't widely considered to have been impactful at all. For example Garviel Loken is such a badass and loveable character that he makes the reader forget about Sejanus really quickly, even tho the whole HH series starts off with his murderer and how much he was valued as being the Warmasters favorite officer. He probably cannot bear to lose any more Astartes are close to him, to pacify ignorant humans that he considers inferior to them. Especially when he is ordered to bow to humankind and to let them govern, having somewhat of an existential crisis for what was going to happen if the galaxy was truly conquered and the need for Space Marines would be eradicated. He already tolerates lodges which are only found in traitor legions I think, even tho the Emperor apparently disapproved secluded structures inside the Legions. His story has some tragic elements but yeah, I wish they hadn't rushed it like that only dropping hints that could develop Horus character but not giving him time to change his attituded showing the impact of those small elements. But I liked that I probably was as frustrated over how he became a traitor as much as he was with being loyal (fuck you Erebus).

It's called bad writing by Dan. He similarly didn't do justice in know no fear. Seems to struggle writing space Marines a bit.

Lol never trust a guy who's personal icon is the eye of sauron

What's the odds of WarHammer getting on Netflix? With the little, I watched of Love Death + Robots, WarHammer could flourish on Netflix.

Imagine you’re a kid playing soccer. Your dad is playing on your team, and he has paid for world-class athletes to be your teammates so you get to easily crush all the other children. 2 minutes before the game ends, your dad sees that you’re up by 100 points. He tells you that he’s gonna go home a bit early. He can’t tell you why, but he assures you that it is very important. He leaves you with a fleet of luxury sports cars, a team of Olympian athletes sworn to obey you, and a bunch of yachts, planes, and other cool gizmos that literally no child has ever had access to. Do you respond by A) finishing your game and asking him later why he had to leave early B) sell your soul to satan, use the gifts dad left you to murder or enslave a ton of people, then try to kill your dad

Because he was super jelly that he went bald before his dad

Its been years since I read it. But wasn't a big part of his fall to do with finding out the emperor had no need for the Space Marine chapters at the end of the Great crusade? I seem to recall this angered him, knowing that for all of their hard work and sacrifice in the trenches, they were going to be massively decommissioned and replaced with standard humans & Autocrats to rule the newly complianced worlds. Not honouring the Primarchs and Space marines that won the victories, but just the Emperor and mankind. I always felt was the final straw that broke the camels back. I may be confusing it with another book though.

They didn't sell Horus falling very well. Should have taken place over like 5 or more novels instead of 1.5.

I always assumed Horus betrayed the Emperor and the Imperium because he was deceived by the Chaos gods into believing that the Emperor was a chaos god as well. So he had basically been ordered to fight against an evil by that very same evil. Fighting one evil (or in the chaos gods case, 4 evils) while serving another evil. That is why Horus betrayed the Emperor, because he was deceived into thinking the emperor was the 5th chaos god. That’s what I always assumed at least

Which books are you talking about? And are they available in audiobooks?

Justification is Heresy.

Let me save you 12 minutes. Erebus.

Does anyone know what brand of shirt Majorkill is wearing? I want to try in on myself.

Mortarion had the most justified fall. The emperor stealing his kill like that was unforgivable. Plus, Mortarion isn't a hypocrite

He fell to chaos because the imperial ran out of rogain

I wonder what would have happened if Eldrad had tried to warn a different Primarch?

Even though I just read false gods, I read this

To sell the most plastic models while he and his dad split the profits.

Bro I got a beard that'd emasculate a Space Wolf. Why the Throne would I wanna ruin a good thing? XD

Horus fought against the Emperor because the Emperor intended to betray the Astartes like he did the Thunder Warriors.

Major gunna go bald at 30 if he keeps it up lol.

Horus' fall to chaos is indeed the Achilles heal of the Hersey . This entire part of the story must be retconned . It just has to be because 99% of the Heresy is bloody amazing. To say such a thing about a series of books with zero sex or adult erotica in it is bloody unbelievable especially with slannesh being active and turning the Emperor's Children. The Astartes have no cocks so I get that but what abut the Administratum and Admiralty and human troops assigned the the Emperor's Children. They could have sexed it up like they did in the Malus Dark Blade books (which is in fact my favourite above the Heresy) That would have been icing on the cake.....for me anyways.

Yes

I thought that Lost Primarchs would have also played into Horus going traitor. He did get into a fight with Malcodor over it.

Emperor: "Horus, My son. You have proven yourself trustworthy to Me in the Great Crusade. I trust you to take My place as Warmaster while I return to Terra." Horus: "Thank you Father. I am honored. I assume You are returning to Terra to stabilize and strengthen the Imperial government and bureaucracy with Uncle Malcador right?" Emperor: "Ohohoho! I have far bigger plans than just that, Horus!" Horus: "I see. Will You tell me about it?" Emperor: "Sorry son, I can't." Horus: "Uh, why?" Emperor: "Because I don't trust you."

"You had more scoops of ice cream than us!.... Than me!" - Horus' real reason why he rebelled against the Emperor

Horus' fall to Chaos has always, to me, been one of those events where they first said "okay, we HAVE to get from point a to point b, knowing that point b is stupid", and then tasked somebody with fleshing it all out. In a lot of ways Archtraitor Horus felt like what I call a Saturday morning cartoon villain; he doesn't have a reason to be evil, he's evil for the sake of it, because he was written to be. He's a mustache-twirling monster serving alien, Lovecraftian horrors, who are also Evil simply because they are, and it just feels like someone later said "that's too 2-dimensional; we need to add more character, and details, and motiv to this choice", without wanting to acknowledge that Chaos, at its peek, is unknowable, Lovecraftian horrors. I don't think there can't be a good reason for Horus to fall, so much as he was written as having fallen, and doing terrible things, for seemingly no reason, and then later had to have "good reasons" tacked on by another group, who drew the short stick, because the Chaos God's don't have redeeming qualities. When he was was written, he was Gargamel, going after Smurfs. Even from season to season, they changed why this clumsy would be sorcerer was after them; eat them to make magic stronger. Eat them to gain extended life. Grind them up and extract gold from their remains. It didn't matter why; the point was he'd try, and he'd fail, with hilarious results. I feel that WAS Horus; the bad guy, just because, and then it was decided, as various elements were being fleshed out, we needed a reason for the loyal son to turn, and they've tried to come up with it, justifying the Drop Site Massacre, the murder of Sanguinius, and other terrible things Horus may have done, if the new HH novels don't totally change it, like Sanguinius fell, and he actually had to be put down, or something silly.

2:18 The Three Stages of Horus Lupercal: 1. Luna Wolf 2. Warmaster 3. Arch-Traitor

Majorkill is a genius... The entire Horus Heresy happened because a bald son hated his father's long mane of hair... Settled... White Dwarf, eat your heart out...

Leman: “Magnus please pick up.” Magnus: “Can’t talk, too busy being sad.”

He got greedy and failed to keep his ambitions in check . Even Kor phaeron told Lorgar Horus was too prideful and ambitious for his own good . He didn't really have a valid excuse since he did worse things than he claimed the emperor did or would do .

Horus also - when talking to his remembrancer - confessed the doubt he'd always had about his appointment as Warmaster, privately thinking that he was in some way inadequate for the position and that the honour of Warmaster should have gone to Sanguinius. "Sanguinius. It should have been him. He has the vision and strength to carry us to victory, and the wisdom to rule once victory is won. For all his aloof coolness, he alone has the Emperor's soul in his blood. Each of us carries part of our father within us, whether it is his hunger for battle, his psychic talent or his determination to succeed. Sanguinius holds it all. It should have been his..." - Horus Lupercal talking privately to his remembrancer.

My headcanon is Horus learned through Chaos the fate of the Thunder Warriors, realized he and the Astartes are disposable tools, rebelled because *what else* is Big E Lying about etc and in true grimdark fashion: Horus' fears were 100% warrented.

Horus was also in fear about what would become of him and is Legions, when the great crusade would end. He was forged for the battlefield and feard that, in an not so far future, there would have no use for Warriors like him and his Marines. He couldnt see a meaning of life in peace. As the Emperor left the Crusade, Horus also saw that this could be a Sign for how meaningless the Crusade could have become for his father and so did he too. I hope my english makes any sense :D

7:41 TTS Magnus: "Father, something terrible has happened-" TTS Man-Emperor of Mankind: *"MMMMMMMMMMMAAAAAAAAAAAAGGGGGGGGGGNNNNNNNNNNNNNNNNUUUUSSSSSSSSSSSS!!!!"* TTS Magnus: "Oh.. oh... I ... I better go!"

One thing I expected when I first started reading the HH was more to be made of the decision to make Horus war master. Growing up I swear the lord made a big deal out of this causing division and jealousy etc and was a big part of the cause of the HH but the way the books tell it I don’t think it really does? Obviously it’s important in terms of the practical side of the HH but it seems like the real issue was the emperor going back to Terra without explanation rather than elevating one primarch above others? I’ve not read early HH in decade so maybe I’m wrong

5:27 When Father and Son go for a walk in the Great Crusade, they asked their enemies how's their health plan. Apparently it was great.

Imagine being bald. Can’t be me.

I actually sort of feel bad for Horus, he never wanted the war master role, as he liked peace, and was put under a lot of stress, and the chaos blade and stupid fucking Erebus used that against him, when chaos left him he was really sad about everything he did and I feel like it was almost an alternate person that had control of him during the Horus Heresy, created by his stress and influences of chaos and Erebus.

Numerous Primarchs/Astartes, including Horus, saw themselves as the peak of human evolution, that it would be in Humanity's best interest if they were to assume political power. Some like Abaddon had patronizing views towards mortals. Others like Argel Tal viewed themselves as freaks and hated the Emperor for having destroyed their humanity. The "conflict" humans / transhumans is at the Heresy's heart. Once again Humanity's weapons were turning against itself. On top on that, Horus indeed resented the military being put under the civilian administration. He saw the army as having the ultimate legitimacy in the Imperium's creation. After the 2 legions' erasure, Horus understood deep-down that Astartes/Primarchs were mutants and monsters to the Emperor, necessary abominations that would be discarded after the Crusade. The Warmaster would never accept that. Erda had warned the Emperor about using the Primarchs as mere weapons and denying their humanity. Most primarchs hated war. They saw the galaxy-wide bloodbath as a sacrifice that needed to be made. But when they believed the Emperor was making them do that for his own ambition, well.. The Primarchs were probably far more human than the Emperor had anticipated, probably more he himself was at that point. Horus was not corrupted by Chaos on Davin. Erebus just showed him exactly what he wanted to see..

The lack of a statue, the lack of an explanation from the Emperor for leaving the Crusade, the idea that shits like Malcador would lord it over him, his sense of insecurity at being up to the task of finishing the crusades all wormed away at his confidence. Plus we know that two of his brothers displeased the Emperor and were cancelled. I can only imagine he felt he was next...

BRO WTFK. There´s a watamelon on 11:04 under the holy emperor sword! I knew it! Watame is truly a blessing of god! (didn't knew major kill knows about hololive a well coming surprise for sure!)

Agreed with that starter line. Not a huge fan of how they fell into chaos. In the stories, the primarchs barely reflect the qualities they are supposed to have. But it is hard for a writer to write about something that is supposed to be far more intelligent than him. But considering them to be so far beyond mere humans and most machines even. Magical even. They would indeed think very differently than most and their lives would take place on a whole other level that few can imagine. We are talking about Personally, I think, Horus's fall was good if they reworked it a little. because he was in fact dying. Even if he suspected deception. The instinct of wanting to live could have been the "genetic" error in this great, semi-god-like entity. Instead of accepting death. he chooses to live and make a bargain with dark gods. It couldn't have been mere deception considering how smart he is supposed to be. In addition, Horus's mood swing after losing a viable alliance because "someone" stole that warp sword. wasn't to my liking. A word bearer manipulated Horus too easily. And Fulgrim. The embrace of art and perfection as ideological tools already was a weakness in their legion and characters. Though at first noble. Subtle changes took control eventually. And I find it easy to believe that Slaanesh would have a not-so-hard time at corrupting Fulgrim. Before he knew it, a daemon took control of his body. Curze was mad to begin with and to my account, did not really fall to chaos but choose that side. He choose to exit the material world because, in his vindication, he knew he was merely a pawn on the board. Being the same evil they sought to quash out of humankind. That is why he burned his own world. It had proven him right.

Would love to see your take of if Guilliman forged an alliance with the Farsight Enclaves.

It went all down hill the day Horus lost his lineup. It would break any man.

The Heresy can be such a mess, a bunch of nonsense mixed with awesome stuff, I can't imagine the kind of effort it would take to reboot even if just partially this whole storyline in a way that makes more sense. I would love to see a timeline regarding the disappearance or "death" of the surviving loyalist primarchs after the Heresy, I think it would be a very interesting topic, what is hard fact and what is legend about it.

Horus turning against the Emperor is one of the more complex and ‘evolving’ parts of Heresy lore. What we see in the first three novels of the trilogy is laid out in a fairly simple, but not very convincing, way. Later, as they added and expanded, it becomes much more muddled. We learn that Horus has uncovered things about Malcador that had him considering the Emperor’s right hand man a monster who couldn’t be trusted, and there is no way that didn’t impact his thinking on the Emperor. We have his pondering done what would become of the Primarchs and Astartes after the Crusade, and there is no way he wasn’t smart enough to see the Emperor’s planned purge coming. He also pondered how the Emperor planned to keep the Imperium together, and from devolving into endless revolts. These things added together make him accepting the future he’s shown make more sense: it’s the future he was already piecing together might follow a successful end to the Crusade. His anger at the lack of his own statue makes more sense here too, as it becomes more of a confirmation that, for all of their efforts, all of his efforts, for building the Imperium for the Emperor, the Emperor probably was planning to delete many of them, just as he had the Thunder Warriors. The visions he was shown thus become not something out of the blue, but a full, in his face, dose of things he already feared made manifest. Even if he didn’t believe the vision, it didn’t matter, because it was showing him a mirror of what he was already starting to think. (And, let’s be honest, a future that some version of would probably have happened no matter if he rebelled or not.) He *also* somehow, at a time and place we don’t see, learned some secret of the Primarch’s creation that led him to further distrust the Emperor, who he accuses of ‘stealing power from the Gods and lying to (his) sons’. Which… the Emperor did. Horus made many mistakes before and during the Heresy, but looking at all of the lore in total, why he did what he did becomes more understandable. He started using Chaos, not as an end, but a tool, and remained deeply disturbed by it even long into the Heresy, as shown by his conversation with Russ. His mistake there was not understanding that Chaos wasn’t something he could control, but that’s a failing he shares with many others. The shame of it is that much of the explanation for why Horus was in a place to turn, and for how what happened actually played out in his head, wasn’t really in place when the first books were written, and so we get shown a sequence of events that only really make sense with context that didn’t exist at the time. A much smaller note, but the… ah, ‘alternate interpretation’ of Mortarian's fall shown in the Siege of Terra books *massively* changes both the character of that fall and much of the complexities of Mortarian as a character.

A better line of thinking would've been this (and it's only a slight shift): The Emperor had pounded it into his head that "the truth" was the justification for all the killing of fellow humans that Horus did during the Great Crusade. Thousands of civilizations and billions of people were killed by Horus because they refused to give up on their worship of gods. Once Horus learned of the Chaos gods and the future worship of the Emperor, I wish GW and Black Library had lean hard on Horus feeling guilt for all those unnecessary deaths. Killing billions of people because you were lied to would've been a better reason than just "the Emps lied to me!"

I think you need to reread the first 2 books. They pretty much show that the kind, merciful, and receptive Horus was a mask Horus wore. He didn’t care about the deaths of the interex, only that his brothers would gain more glory while he was stuck. The mourneval weren’t actual advisors, only foils to make to make him look open to advice, benevolent, and wise. His remembrancer only got close due to him due to her similar pompous ego and overblown superiority complex. Horus lived everyday in fear that Malcador knew who he really was and the high lords of terra realized how much of a fraud he was. So all erabus had to do was show the emporer’s hypocrisy and a future where Horus was forgotten.

I fucken love the way he uploads new video every day. ( Miss Timmy 😅)

Imho, Horus' and Mortarion's falls were the worst executed falls to Chaos in the HH novels.

The Great Crusade was a massive undertaking for anyone to accomplish, and the Emperor was the head of said conquest of the Galaxy, but leaving Horus to the conquering of the Galaxy should have been a cake walk for him, though the Chaos Gods made that decision seemed bad in hindsight.

I haven't read the books, but wasn't it the case that Horus supposedly spent hundreds of years worth of time doing "labours" for the chaos gods in the time he was in the warp with them? I know I read or heard that somewhere, but have no idea where and it could be complete BS. But that would explain why his character change was so sudden. Because those few moments had been hundreds of years of creeping corruption from his point of view.

I've got a different theory. I think there was a realization after Horus told Magnus to fuck off - I think he realized what he was (a minor warp god/entity) while in that state and no longer saw himself as being a son of the emperor but rather as a potential equal/competitor. While reading the books, the shift feels jarring but they always left it open to the idea that during this entire thing, Horus had a realization but they never described what it was. After all, if he had followed Erebus, he would have joined Chaos right away. It was a "no I won't join Chaos and be your puppet but this whole situation is fucked so I'm going to do my own thing" type of deal. At least that's my headcannon.

you have to understand the hours never knew that he would survive is he didn't accept aruba's. Horus specifically says in the novel that he didn't care if the vision was a lie or not. He was going to make the decision weather he lived or died or not. It's not like he knew that the chaos gods were going to turn him over to their side. He honestly thought that by choosing to live he could create an imperium where the emperor couldn't achieve his goal of God hood. He only saw the one side of the potential future. Regardless of if it was real or not. He would do his upmost to go against such a plan even if it turned out to be false. The man was literally afraid of his legacy being destroyed if he didn't accept the offer for life at that moment. Even if another primarch in the future survived the Knife. It's not like he knew that he could potentially live. For all the man knew He would quite literally hit the bucket if he rejected. i mean if you are on deaths door and someone offered your life at, you're lowest moment Wouldn't you accept. It's not like you could know at that moment that by rejecting the offer you could potentially live. The man had problems yes, and he was under a lot of stress. But ultimately, he wanted to do the best he could at that moment, and horus was damned to be controlled by some so called "Gods in the warp" For their own games Horus knew about Choas but was ignorant about how prevalent that actually were. It's like a child knowing that a gun could kill but only realizing how deadly such weapons are once shot.

One thing you overlook is the whole thing that happened on the flagship with Abaddon and others killing the civilians in order to bring his wounded body to the medics. He was already sick and tied of the normies, but the fact that his sons, his astartes was to be punished by them for trying to save him, also was a big push towards killing all civilians. And when one ball is rolling the rest follows.