Another one for Eris for being my bisexual awakening but ALSO giving me a love of wispy smoke effects. But that's more on her looks than her villain-ness, though I love that she beats herself by expecting everyone to be like her.
Another one that comes to mind is The Killer in Persona 4. They have... No motivation. It's because it's fun. It's because they could. It's because it's something that's only they could do(as far as they knew) and gave them power over someone else. The killer's pettiness is so disturbing and at the same time so real, and everyone admits that there's no inherent evil in the character, it could happen to anyone that has flimsy moral limits and has no real objective in life.
Actual Persona 4 Spoilers
Also, Adachi saying he only became a cop is because he could get a gun (and was possibly moved to Inaba for sexual harassment) went from "kinda real" to horribly real in the years that followed the game. He's no longer the exception(in some places, never was), being Adachi is the norm for most police infrastructures, but rather than being a cabbage hermit in their own jurisdiction they're "a family man" in the eyes of people that could stop them.