I've been considering what it is to make an "evil" character lately. I remember once a long time ago I used to have discussions with people about what constituted as "evil" when you made a character. At the time these discussions started it was the "obvious" evil verses the more subtle ones. (Think drowning kittens vs taking over a kingdom.) Now in the case of drowning kittens (or people as the case may be), most people would consider that a more obvious type of evil. Murder = bad, if you don't know that it also = evil. So what about the other one? Taking over a kingdom. I use the idea of the kingdom but it could also be country or anything else that requires a leader.
If you look at history it's always, always the victor who dictates how things are seen because they're the ones who will write the history books. Imagine what would have been written about America if Germany had won WWII... It wouldn't have been pretty I'm sure. The atrocities that were committed by the furor and his followers would have been justified in their minds and in the history books that they wrote after. Fortunately for a great, great many people those atrocities are not only seen as such but there are a great many efforts by many more people to ensure they do not happen again. The world has moved on and away to try and understand how to accept all peoples, no matter their differences. We're not there yet obviously, but we're working on it and that forward motion is a good thing to my way of thinking.
What I find really interesting is that I bet a lot of people would hesitate over saying whether or not invaders are "evil". Invasion of another land or kingdom or people often includes a great deal of death, torture, destruction of land, maybe even slavery, theft... huhm... alot of those things are lumped in the evil category aren't they? We seem to think of them as "necessary" evils, and it's all in the presentation. In a fantasy novel "invasion" much like in real life can happen lots of different ways. If you're portraying the take-over as a response to a different invasion force, it's much more acceptable. The person invading is often (if not always) seen as the bad guy and those defending or fighting against the invasion are the good guys.
Yet the ways you could write such a thing are limited. As interesting as the bad guy might be to write about, very few people actually want the bad guy to win in the end. Why would we want the bad guy to win in the end when that happens far too often in life? Yet look at the history of the world and think about how many things would be different if someone else had won the battle. Would we really be more "evil" than we are now? I'm not so sure. So I've been considering my novel and what I want to do with my books in this light and I realize that I always seem to make my bad characters redeemable in some way shape or form. Now I like that because I believe all characters must have motivations that are true to the character and the choices they make in the story (much as we are in life). By keeping in mind that the "bad" guy has his own motivations and believes in them just as strongly as the "good" guy it's harder to see who the "bad" guy is. Hence the word Antagonist.
When you've got someone in the story who presents an obstacle for your protagonist they don't necessarily have to be evil, they just have opposing goals to your protagonist. I think that's one thing that I'll always enjoy exploring through my writing. What is it that makes a person evil, vs good, especially in gray situations where the outcome makes all the difference? I've got my own theories about it, but it's still interesting to explore. It goes along the same lines for me as "why does God allow bad things to happen?". There's always reasons for things in life, I believe that even when it's really hard to see or acknowledge, and I utilize the same thing with my writing. My bad guys so far always believe in what they're doing just as strongly as any of my good guys, though perhaps the biggest difference is the good guys try to do things selflessly while the bad guys are often selfishly motivated.