For me, villains are the most difficult characters to write. I find that I have to spend alot more time developing their character and carefully editing their dialogue than I do any of my other characters, hero/heroine included.
It’s difficult to nail down a convincing “evil”. The world is filled with cardboard villain cutouts.. people who want to take over the world, but we’re not really sure why. Evil is so difficult to write convincingly that alot of people just default to crazy or mentally disturbed. There is an evil that is perfectly sane and doesn’t stem from some hideous childhood event. It hasn’t lost its moral compass, it consciously, willingly set it aside.
That is the evil that really intrigues me. Naturally, I’ve yet to commit it to paper in a way that satisfies me.
Another type of villain I am intrigued by is the “true believer”. This villain has a moral compass, but perhaps a skewed one. Someone who is committed to their cause through feelings of personal justification will often go much further to reach their goal than someone simply doing it for selfish reasons. That can make for a really formidable enemy.
People are excellent at twisting their own morality around their circumstances to best fit their interests–the trick is in convincing your villain that he is justified in his actions, for better or worse.