The question is: why? Most of the over-the-top psychopaths in books that land in my "maybe I should review this" pile quickly move into the "not going to read it ever, even if it was the last book on earth pile." Why? Because the bad guy has no motivation for being as bad as he is.
LJer Medwriter asked me to write more about how I think everyone is the hero of his own life. People have to have made some kind of peace with what they do. They might do bad things because they truly believe it is the lesser of two evils. Or they feel it's justified in some way. Or just like a dieter who's been given a pound of Jaques Torres chocolates for her birthday, they have gone through a cycle of craving and denial before they gave in to temptation (only it might be to sexual abuse or serial killing, not chocolate), and then afterward they feel gross and guilty, and then the cycle begins again.
To understand your bad guy better, write a passage in third person that shows the bad guy in action. Make the reader hate him.
And then write the exact same scene, first person, from the point of view of the bad guy. I did this with a boss once, and it was very revealing. Suddenly she made sense!