Honestly, if the MC wants a realistic peaceful era as a result of his violent conquests -- he needs to completely remove the upper echelon of all societies, and disband their armies. It's not good enough to just "smite the evil ones", he needs to take over the monopoly of force, and let humans focus on their daily lives.
Otherwise, there's always a chance for conflict to spark up. There's no peace in war. It makes absolutely no sense for the MC to want people to fight him perpetually to infinity.