Revenge is one of the more common motives for murder. Much less comprehensible and much more dangerous are killers who are not content with eliminating the object of their hatred. Seeking out anyone who reminds them of the enemy, they kill again and again. "Mad Dog" McCafferty killed three people to avenge his son's death. Anatoly Onoprienko, abandoned in an orphanage by his father, sought out revenge on any happy family he encountered. What accounts for violent vengeance? Dr. Stone probes the science of revenge to profile these killers, looking for clues that might shed light on their motives.