I just saw the whole thing as him cutting a deal with the Emperor to use his talents for their cause, and once the Emperor croaked, he felt that he could try taking power for himself. I don't think he was brainwashed, I think a detail like that would've been clarified in the game. Perhaps he was doing it out of some misguided sense of it being for the greater good, but I think he would try explaining that to his family before fighting them.

As for why he had a change of heart? Well if I recall right, Demon King!Emperor was trying to kill him too, so it could be just him acting in his own interest at first, but then having a genuine change of heart after traveling with his family once again. Or maybe he realized that it was futile to try taking over the world since he would get inevitably defeated, I mean if even the King of Hell got defeated...