Interesting responses. You're an interesting and also polite lot, I appreciate that!
I'm curious as to why people think sociopaths / psychopaths are evil. By definition, sociopaths are unable to feel emotions as much as normal people. Sometimes they may feel no emotions. But a key aspect of being human is emotions. It's part of how we interpret the world, and clearly emotions somehow evolved in order to help us better exist in the world. Empathy causes us to take care of each other, for instance, promoting the continuation of the species. And if we are not empathetic and even sympathetic, we don't have any reason to take care of people - which could certainly cause population to drop off in fairly obvious ways.
Yet I don't think that necessarily the case of sociopaths. An estimated 1-5% of the general population is sociopathic. Are they predisposed to be "evil?" I think that case could certainly be made. Emotional people can be evil, too. Hitler (to use an exceedingly abused example) for instance was emotional - sometimes too emotional, but he was also screwed up from an incredibly abusive father.
For my part, I know I'll be trying to craft as complex an evil person as I can muster. I'll also be toying with a "rebound effect;" where the actions of the antagonist cause the protagonist to consider venues of action he would normally consider unethical ("evil"), and maybe the protagonist will cause the antagonist to change somewhat as well. I want to tell a good story, but I also want the readers to be asking questions on their own - questions that I can't answer, and that maybe no one can.