Guts died to the Critical Hit Atrocity. Atrocity ignores durability of all types and is super effective to anything. The Berserker Armor just let him use the last bit of his blood to make one more swing, even if it was fatal, but Guts dying standing up and Dimitri willing to protect Guts corpse from the demons who will be trying to take Guts's soul since demons are attracted to his Crest like moths to a flame, out of respect is a real good ending.
Think of it kinda like how Pokémon moves can be super effective, like water being good against fire. But imagine there was a move that was super effective against EVERYTHING! That's what Atrocity is.
