The most gripping stories always have a villain. It may be the hero's own weaknesses (or the heroine's), it may be wild animals or nature, or it may be a person, but there needs to be something keeping the hero and heroine from achieving their happy ending, or the book would be all over by about page ten.
The best villains are ones clearly explained, fully developed characters that the reader can't help but want to hear more about. No one really cares about the villain who is so totally evil he's practically a cartoon character. In fact, the best cartoon bad guys are pretty awesome themselves.
Your villain may be just an ordinary Joe, caught up in evil. Or not smart enough, or just too nice, to see he's being misled or used by someone else. He may be stuck in a tradition, not seeing that times have changed. But even if he is pretty bad, he will still have redeeming features. Even ax murderers love their wife, their kids and their dog. And the more followers he has, the more the villain will believe he is right. He has too much invested in himself and the followers to change easily. Besides, they probably keep telling him how wonderful he is and how right his plans are, making it ever harder for him to see another possible path.
If your villain is insane, make sure you have your facts right. Don’t give him some malady that's easily treatable with a bottle of pills, or your plot will be too weak to make the story good, even if "the voices in his head" tell him not to take them. A smart heroine could feed them to him in his morning coffee!
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