I think maybe there are mainly five flavors of villains, six tops.
1. Those that are in it for the money for the most part. Truly professional criminals in that this is indeed what they do to make a living. Think Captain Cold, or Deathstroke.
2. Zealots. Villains that believe in a cause or ideal and don't care who gets hurt in their attempts to make this a better or "proper" world. So think Magneto or Kobra.
3. Those in it purely for the thrills. Devil may care types that chafe at society's strictures and mores. These are usually thieves generally, with the occasional "muscle" types, so I'm thinking Catwoman, The Black Cat ect.
4. The totally insane. The completely schizo ones like Norman Osborn sociopath or the Joker with his so off kilter psychopathic "philosophy".
5. The somewhat more controlled Sociopaths like Luthor, Dr. Doom or Doc Ock. Men and women with huge outsized egos that believe their only crime is being superior to every other human being on the planet.
The sixth might have characteristics that are an amalgamation of the previous five. They might be rookies, or they might be vets in the super villain game. Either way, they are in it for one reason... To BE super villains. To live that life. To be the center of attention. To be a player in the grand game between Super Heroes and Super Villains. To be granted a certain amount of respect, fear, honor, infamy and yes, the finer things that go along with being an "arch criminal." I think for a lot of villains this is the real problem with quitting. They'd have no sense of identity without being Captain Dastardly, menace to society and sparring partner to Wonderousman. Whether it was succumbing to the impulse of revenge, enjoying the perks, what ever... They couldn't quit now easily even if they wanted to.
