Among the many pieces of advice that writers hear repeatedly, one of the most common is: Don’t make your antagonist pure evil. Variants of this advice are Your antagonist should be a full character, too, and Antagonists don’t think of themselves as “bad guys”, and Give your antagonists realistic motivations for their actions.
Personally, I love my antagonists. It’s rare that I come up with an antagonist that isn’t delightful to write about – I look forward to working on scenes from their point of view, sometimes more than scenes with my protagonists. With my upcoming NaNo book, in fact, the current protagonist was the antagonist, originally.
I’ve heard many an actor say in interviews that their favorite characters to play are the bad guys, that it’s fun to unleash creepy and disturbing behavior in a context that isn’t going to hurt anybody. Well, I’m lousy at learning lines, which is why I’ve never seriously pursued acting, but I have a similar attitude with writing “bad guys”. Just have fun with it. Come up with someone who’s really twisted and let them loose on your story. And by twisted, I don’t mean this person needs to be evil, crazy, or monstrous. They don’t need to be vengeful or angry. Just warped. Take the same motivation you’ve given your protagonist, add a wrong turn in the logic process, and BAM, you have a great start to your antagonist – and a nice little parallel going on between your “good guy” and your “bad guy”.
And while it’s fine for comic books to explain that the reason a supervillain is so horrifically screwed up is that he got dumped in some acid or was the victim of a lab experiment gone haywire, you probably want a better background for an antagonist in a novel – even if you never give the full back story in the text of the book.
Think about things that real people go through – people you know, people on the news, friends of friends, anyone – and think about how much it really takes to make a person crack. It’s a lot. If your antagonist is an outright villain, it took a lot for him/her to get that way. What did that to him/her? When you, as the writer, feel sorry for them on some level, you’re getting somewhere. Your readers may never feel sorry for your villain, and maybe they shouldn’t. But the writer always has to know more than they’re telling!