I find that villains, whether mine or another author's, are much more realistic and effective when they have a reason to be evil. Even though they work in some applications, evil masterminds that are purely evil and have never been anything but evil aren't very deep, because there's nothing to relate to. There's no humanity for the reader to grasp, no way for them to understand how they got to that point. While I don't know any personally, I'm willing to guess that evil masterminds aren't born with the desire to rule the world and crush all humanity from the time they can walk.
We all have a sinful nature. But most of the time, there's a reason why we sin. It's not an acceptable reason, to be sure, but there's some struggle, some desire, some factor that tempts us to sin.
So how did your villain get to be evil? What is he after? These are the questions I like to answer about my villain, so I can understand why he got here.
Building realistic villains can be hard, at least for me. As I try to build my villain and give him a reason for his evilness, sometimes he comes out too soft... or he's so pitiable that he's not frightening anymore. I have this problem with Mark in Alaidia. He's a traitor/crossover character, so he's not evil at his core but he still follows through with some dastardly plans. THAT was very hard to explain. His uncle Edric, my main villain, was easier, because he is the typical seize-the-throne overlord type. He married the eldest royal child and wanted to steal the right to the throne from the firstborn son. Pretty simple, and sometimes an evil mastermind is what works best.
