Dennis Rader is the infamous serial killer known as BTK (Bind, Torture, Kill). A new documentary on A&E tries to make sense out of how he became a psychopathic killer. Forensic psychologist Katherine Ramslend has been talking with Rader by phone for over ten years, trying to understand him. She says that every serial killer has a unique set of factors that turns them into monsters. She pointed to his chilhood, when he began to trap and torture kittens before killing them.