Why do we kill? Why do some of us kill, and others resist the temptation? What makes a serial killer? Dr. Dorothy Otnow Lewis, a psychiatrist who’s worked with numerous serial killers, including Ted Bundy, Arthur Shawcross and Joel Rifkin, has been looking to answer these questions for decades. Fascinated by the human brain and its capacity for ruthlessness, she has spent her life investigating the interior lives of violent people. With each case, she came closer to developing a unified field theory of what makes a killer. Along the way - steering away from the conventional wisdom of her colleagues - she explored the world of multiple personality disorder.