In his excellent book Fiction Is Folks, Robert Newton Peck has a chapter called "Black Bart Sniffs a Daisy (or something to that effect. I loaned my book to a writer friend so I can't look it up)." The chapter is about giving your bad guys some redeeming qualities. After all, they weren't born bad.
Let me say I have great respect for Mr. Peck. His book is one of the best, funniest and most memorable books on writing I've ever read. I would never presume to argue his point that many bad guys are two-dimensional, being bad just because the good guy needs someone to foil.
I draw a distinction between bad guys and evil characters. A bad guy is the enemy, the Antagonist, if you will, who for whatever reason causes the Protagonist great angst. The bad guy is an obstacle to overcome, and as such can be good, bad or indifferent. He just needs to be in the way.
But evil characters...Those are a horse of a different color. An evil character enjoys being evil. Maybe she didn't start that way, but she's evil now and loving it. She likes causing pain and chaos and grief. She gets off on it. Drusilla from Buffy the Vampire Slayer springs to mind. Yes, she was once a virginal psychic, but she is now one crazy-evil queen of the undead. You cannot appeal to her good side; she doesn't have one (unless you count Spike). Falling into her talons is a drawn-out death sentence. If you're lucky.
I've always been fascinated by those behind-the-scenes interviews where the actor playing the villain explains why the villain is just an okay guy with a misguided vision, a victim of circumstance, really. I have never heard an actor say, "Yeah, he is one evil dude, and I love playing him."
TT: This supports my theory of why many actors are Liberal Progressives. They over-identify with people to the point of excusing any behavior as misguided. Any behavior except conservatism. That is condemned as intolerant and bigoted. Oops! Wrong blog.
Most of my bad guys are evil characters. Like Joey says, "If you're gonna do it wrong, do it right."
My nasty ladies love the violence. My evil dragons eat first and ask questions...well, never. People do reach a point where evil is an end in itself. The quest for power or pleasure or knowledge becomes all-consuming and that's when you don't want to be between them and their goal.
Of course, that's exactly where my Heroes end up.