Very often, a positive comment a reviewer will have about a book is that the characters are "complex." That means that the characters are three-dimensional and surprise you occasionally. A tough cop who is afraid of public speaking or a fragile young woman who finds courage to stand on her own are characters we like because they have depth and are not pure stereotypes.
Among writers of Christian/Spiritual/Biblical speculative fiction we often think about the complexity of our heroes. Perhaps as a respect for literature and perhaps simply as a reaction against the too-good-to-be-real-sickly-sweet heroes and heroines of traditional Christian fiction we have taken pains to include weaknesses as well as strengths, show the character as being vulnerable and even sinful at times without condoning the sin simply acknowledging it as part of every Christian's struggle.
Unfortunately, in both Christian and secular speculative fiction, I don't always find the same care given to make the villains. There are, of course, exceptions. Darth Vader's journey from Anikan Skywalker, Jedi Knight to the general of the Dark Side has been chronicled over 6 movies and 30 years. Gollum and to a lesser extent Gandalf's mentor who are both seduced by the power of the ring have some of this complexity.
But those are exceptions rather than the rule. Most fantasy, horror or science fiction villains are portrayed as simply being evil without much since of where that evil came from. Likewise, they are all evil with no good side at all. It's almost the flip side of the too-good-to-be-real hero. Even if a villain does something good, usually we attribute evil motives to the behavior whereas, we will tend to excuse or at least empathize with the failings of our heroes.
I don't know why this is. Well drawn, three dimensional villains are as engaging to a reader as a three-dimensitonal hero. Perhaps in the Christian world that may be the problem. We may be afraid that if we understand the bad guy it is like excusing him. This is nonsense, of course. As a someone who went through high school being assaulted both physically and verbally in every way imaginable, I can understand the actions of high school shooters, yet I can't condone them. Most of us who are abused in the way they are barrel through, get therapy as adults, and don't go shooting people. But that doesn't mean we don't empathize with them. The only difference between me and a high school shooter was a choice I made. It was a choice influence by parental love and a relationship with Christ, but it was still a choice.
Wouldn't it be nice to see villains and heroes where the only difference between them is a series of choices? Instead of the inexplicably inherently evil villain, we have a villain we know is like us. That is the scariest villain of all.
Maybe that is why we don't write such villains. I don't want to admit that I could do the things this evil being does. Yet, I may be just one choice away from beginning the journey down that path. If I see a villain who enjoys coaching a little league team on the weekends after a hard week of running a crime syndicate, I call into question the whole inherently evil assumption, and I have to face the reality that at this moment I may be a generally good person who sins occassionally, but a few bad choices later, I could become an evil person who does good things occassionally, and I'm not totally sure where the line is dividing the two.
Still, such understandable villains are important as anti-role models warnings that our choices determine our destiny and that any one of us has the potential of becoming an angel or a demon.