There's an interesting post up at Sarah Hoyt's blog about motivations for villainous characters. She takes the position that no one is consciously evil, and evil behavior is usually motivated by a desire to do good or to fit in.

This is a very common view in science fiction; it's practically the default. With some notable exceptions, science fiction has antagonists rather than villains. It's part of the mindset that goes with writing science fiction: problems are to be solved, the universe is comprehensible, and things follow rational laws. If someone opposes your main character, that person has valid reasons (though they may be based on faulty information).

Evil, with a capital E, is a fantasy concept. It requires the existence of a supernatural yardstick marked with Good at one end and Evil at the other. It requires an absolute standard, and one of the strengths of science fiction is that it treats standards of behavior as infinitely variable.

(This is also science fictions's Original Sin: it treats human behavior as something which is controllable, and therefore the discussion becomes how and by whom it should be controlled. And that's a very slippery slope above a very deep pit with bodies at the bottom.)

However, I'm afraid I must disagree with Mrs. Hoyt about this. There are people who knowingly do evil. Consider career criminals like Mafia members. They're under no illusions that what they're doing is right or good. I recall reading one account of a Mafia soldier who had plenty of his own money but preferred to take his girlfriend out on the town with a stolen credit card. Screwing someone else over just made the whole evening more fun.

Maybe this is a sex difference. Part of being male is the desire to be the top male, and that is accomplished in part by stamping down all the others. Being the baddest dude around is fun. There's a solid biological basis for this. When you believe you're high status, your body makes more serotonin, and that makes you feel good.

To a very great extent our whole concept of good and evil developed to preserve the social group against the innate drives of its individual members. Our brains invented right and wrong to help keep our bodies in line, and societies where the brains won prospered and perpetuated themselves.

But those drives are still there, and there are still plenty of people who indulge them. And plenty more people who fantasize about indulging them (because we all chafe at our roles in society), which is why a good villain can make a story.