K.S. Crooks
Maester
The main thing is that villains aren't villains in their own eyes.
They have full justification - and the reader should be able to see that self justification - and the reasoning behind what they do.
If that justification is really weak - then they are terrible pantomime villains.
The more compelling reasons and justifications for their villainy - the better the villain.
Great evil can be done with good intent.
I totally agree with Terry. Justification for the evil deeds can be very important even if it is not rational. True evil or crazy people/beings don't see themselves that way. I love in the Batman movie Dark Knight, whenever someone calls the Joker crazy he instantly focuses on that and says "I'm not..." he refuses to even say the word crazy, because he sees himself as a visionary. It's nice to have the same options are available to the heroes in your stories to look various ways.
To adult readers a villain that seems in every other way normal is usually more scary. Hannibal Lecter and Norman Bates are far scarier than Dracula or The Wicked With of the West . To children I think the opposite is true and don't even get me started about the flying monkeys.