I like monsters. Like Hagrid in Harry Potter, I think they’re interesting. I’m not a fan of horror movies because there’s rarely a surprise with the monster. The monster is the bad guy, he (or she) (or it) is evil and out to destroy and there’s not much depth beyond that. I much prefer the sort of stuff Joss Whedon does where sometimes the monster is the innocent victim of prejudice, sometimes it is evil, sometimes it’s charming and funny but still does the most appalling things (kitten-eating Clem as an example).
I like characters who are people. I don’t like having everything up front and no surprises. I like discovering that the “good” guy has a streak of darkness in him. I like it when the bad guy has a sympathetic (if twisted) rationale for what he’s doing. I like having the possibility that either one might cross the line.
One of my favourite universes is the X-men universe and I think a lot of the characters have that kind of depth. Not all, but the ones who have been developing with multiple writers over decades have some good backstory to play with. Magneto from the movies is a great example of a villain for me. This is a guy who has been kicked in the teeth by life again and again. He’s been persecuted for being a Jew, for being a mutant. His family was killed by the Nazis, his wife and children were killed (or so he thought). By the time we meet him, he is determined to make certain no one is ever going to attack him again. He’s not willing to trust to diplomacy, it’s failed him again and again. He truly believes that the only way to protect himself is to make sure he’s the one in charge. There is the tantalizing possibility that maybe Xavier and the X-men will be able to convince him of the error of his ways. He’s not evil in the classic melodramatic sense. He has flaws in his thinking and standards of behaviour but there is a strange sympathy to his point of view.
Compare Magneto with Batman. Batman also had a traumatic experience in his life and dedicated himself to making sure it would never happen again. Same motivation as Magneto. But Batman isn’t trying to save the world, he’s only dealing with the immediate situation in front of him (this particular mugging, robbery, whatever). You would think Magneto’s noble and lofty goals would make him a better person but in this case it actually makes him worse. It’s his belief in his noble goal which lets him justify some really appalling means to get there. Batman has a strict line he won’t cross, but there’s always the risk that this particular situation or villain will push him over.
A hero who is never tempted into darkness is boring. A villain who is just Evil is boring. A writer may not always have time to get into all the backstory, but there should be something besides one-dimensional motivation.