One of the things you see a lot in fantasy stories is a villain who is purely and simply evil and knows it. No rationalizations, no semi-plausible rationales, not even a rotten childhood to blame it on, just the Dark Lord Who Wants To Take Over The World Because He Is Really, Really Evil. There are ways of making such a character work – one would be to present him as a sociopath – but they are limited in number, and they are difficult to pull off.
Most of the time, the thoroughly Evil Dark Lord comes off as a cardboard character, because he doesn’t have any reason for what he does except “a lust for power.” And “a lust for power” alone just doesn’t seem like sufficient reason, since there are a great many ways of getting vast quantities of personal and/or political power without murdering and torturing and so on (and those ways have the added benefit that one is far more likely to retain a few friends who actually trust one). Which is why it is so important both to come up with the real reasons why one’s villain is trying to conquer the world, commit genocide, kill the hero, and swipe the very last chocolate chip cookie, and also to come up with the rationalizations that he is using to justify doing all these evil things.
A possible solution for this difficulty is to deliberately look at the other side of the villain. What are his/her good points? What does the story look like from his/her perspective – that is, the same events are happening, but seen through his/her eyes, the “villain” is the real hero (and the eventual success of the “hero” of the original version is a tragedy)? And best of all: what is the one thing that the villain loves madly and would be much too ashamed to ever, ever admit in public (like reading comic books or romances, or watching Weekly World Wrestling, or buying The National Enquirer)? In other words…what are all the things that make the villain a human being, and not just the Dark Lord Who Wants To Take Over etc. (Someday, I am going to write a Dark Lord whose secret vice is that he loves pro wrestling…not this week, though.)
What do you do to make your villains real characters?