Evil Serial Killers: To Kill and Kill Again

The FBI estimate that there are between 25 and 50 serial killers at large in the USA at any given time. But the truth is few people kill. We occasionally say we could kill someone, but that is usually hyperbole. Most of us can imagine what it might be like to be driven to a senseless act of violence in an unendurable situation. To kill once is one thing; to kill over and over again is quite another. What drives these people who kill and kill again? Are they evil or are they mad? Serial kill...