Piers Morgan interviews serial killer Mark Riebe. Mark is one of Florida's most ruthless murderers and is believed to have killed more than 13 women in a period from the late 1980's to the late 90's. He was convicted in 1998 of the murder of Donna Callahan on August 6, 1989, and sentenced to 25 years in prison. Piers gets straight to the point to find out what has driven these people to commit such heinous crimes.