There's a thin line between those who kill and those who don't. Journey into killers' minds as they reveal why and how they crossed to the other side. S1, Ep1 Ice Cold: Levi King When 23-year-old Levi King executed two innocent neighbors in 2005 he finally found a release for his life-long inner rage. For the first time in his life, King felt a sense of peace. S1, Ep2 Mad Maks: Maksim Gelman In 2011 Maksim Gelman went on a brutal killing spree in New York City that left four people dead and four wounded. It began when he killed his stepfather by stabbing him 55 times, and ended 28 hours later in a bloody confrontation with the NYPD. S1, Ep3 Monster Inside: Robert Lopez Robert Lopez warned women he loved never to betray him. In 2005, after serving 10 years for stabbing and nearly killing his first girlfriend, Lopez had a new girlfriend. But when she gave Lopez reason to doubt her affection, he exacted fatal revenge. S1, Ep4 Twisted Love: Dena Riley In 2006, when 38-year-old Dena Riley met convicted rapist Richard Davis, their relationship turned out to be a marriage made in hell. They shared the same twisted desire for three-way sex--followed by murder. S1, Ep5 Payback: Earl Forrest Two weeks before Christmas, Earl Forrest burst into a friend's house and gunned down two people in cold blood. Still thirsty for mayhem, Forrest then ambushed the deputies who came knocking.