What turns someone into a serial killer? How do they go undetected for so long and what compels them to keep killing? Meet the criminal investigators and serial killers they profile. Canadian journalist Nadia Fezzani spent years corresponding with Richard Cottingham, one of the most notorious killers of the 80s. He is now serving a 268 year prison sentence for the torture and murder of at least five prostitutes. Also meet Elmer Wayne Henley, one of the Houston Mass Murderers, who helped murder at least 27 young boys. He started killing when he was just 15. Expert Stéphane Bourgouin interviewed Henley in prison and sheds new light on the case. Meanwhile, in Germany, nurse Wolfgang Lange poisoned several of his patients. After serving a 10 year prison sentence, he is now free and a German profiler met with him for us.