@Fafulin Reincarnated as a Lich. MC was the hero's mage. They beat the demon lord, only to be backstabbed successfully (both he and the hero or healer, I can't recall exactly, died). MC holds a grudge so bad that he became a lich. Despite everything he can't revive the girl, so he decides to become the next demon lord so that all of the human nations will band together to fight him, and stop the infighting. Well... except for that 1 fker who backstabbed him, he gets the good ol vengeance.