"Mark of a Killer" examines the disturbing behaviors of serial killers. Step into the minds of some of the worst serial killers to date as they taunt law enforcement. Some of these killers get a perverse gratification by habitually stealing mementos from murder scenes including things like shoes, jewelry, keys, and even body parts. All of these stolen clues are meant to taunt the investigators while they build up their notoriety -- these killers often lead trademark clues behind as a way to mark something that was theirs but ultimately is their downfall as investigators use these as clues.