Like trying to decipher the value of a book without ever opening it, judging a person by his or her outward appearance can be a mistake. Someone may look perfect on the outside, but even the most kind-hearted soul has a dark side looming. "Diabolical" is a docuseries that delves into the minds of murderers, asking a simple question: "Why did they do it?" The acts were committed with extreme premeditation -- many were months or even years in the making. Tension builds in each episode as law enforcement taps into the methods of devious masterminds, aided by insights from forensic psychologists and psychiatrists as well as criminal profilers.