And I agree with what you say in the second para, as well as that it's good to have a decent backstory for your A-list villains, but having to have some sort of Big Dramatic Psychodrama as an explanation for why they turned to a life of crime leads to unfortunate (if dated) incidents like Young Lex Luthor deciding to become a supervillain because the World's Smartest Teen couldn't figure out, or admit to himself, that Superboy wasn't really to blame for making his hair fall out.

You know what one of the best explanations with regards to villainous motivations was, ever? Supposedly, someone asked Willie "The Actor" Sutton, about as close as the real world has ever come to producing a real supervillain, why he robbed banks, and he said*, "Because that's where the money is."

*Probably not really, but it's still a great line.