Truth was going to kill Regret anyway because he had already caused enough problems. That's why truth called off the reinforcements going to regrets location and bombed the temple regret was in.
ThunderBuddy u r wrong he was not gonna kill reget coz if u know the arbiter Reaper mora me that arbiter redeemed the prophet so they weren't gonna kill him but cheif killed him anyway but fortunately when they destroyed the temple the gravemind catchs the prophet and 343 guilty spark and revived him but kept him for 23 years so he learned his lesson
Samuel Hartley Mate, Halo Wars is many years before the events of Halo 2, a lot happens between the 2 games. Cortana even says in the game, "I think you might be doing Truth a favour." when you go to kill regret. For example, it was because of Regret that humanity could learn the location of the second halo ring.
Best and most safe way to kill him on legendary is to take out those honor guard elites first, then mount the Regret's chair and beat the hell out of him. That way, when you dismount the chair, you don't have to immediately deal with the honor guards which will kill you instantly. After each beating, kill all elites in the room (there's like 3-4 elites each time) before you mount Regret's chair again. It takes something like 5 beatings on legendary. (you'll get a few checkpoints between those beatings, at least I did)
