From stuff I've watched and read it's completely random. Some do it out of boredom, some have a compulsion that is like an itch you can't scratch. A major one that seems to come up a lot is the finality of it. They are basically playing "god." To be able to be a serial killer and not be one off I feel as though you have to be mentally unstable. Most people feel guilty about stealing a candy bar, stealing a life would be a heavy burden to bare unless you aren't all wired correctly.