Villains turning Good

I'm not sure if there's a term for that. I'd almost call is tsundere the way it happens. "I'm only helping you since I don't want anyone else to kill you but me" (Sure, Vegeta ). In any case it when the villain does turn good, it's usually when a greater evil arrives which dwarfs him. Despite all the hard work the hero took, it's back to the grind. The villain is merely a marker for the hero's power and growth. Bleach has a good example with the Menos. First time it arrives it's the biggest baddest hollow ever. While in this case Menos it doesn't become a friend, it does just show how powerful Ichigo was at the time.