Sexual sadists are addicted to the thrill of inflicting pain before killing their victims; an analysis contrasts the psyches of David Parker Ray, the Toy Box Killer, and Dennis Rader, the BTK Strangler.

Some serial killers are driven by a hatred for humankind, while others seek revenge for their abusive childhoods; some simply love to kill; a look at a killer labeled America's most meticulous serial killer, Israel Keyes.

A shockingly common breed of serial killer: doctors and nurses who kill patients; serial killer and physician Dr. Harold Shipman murders more than 200 victims.

Examining the most excessive serial killers who murder, often crossing over into horrible perversions like mutilation, necrophilia and cannibalism.

These are serial killers who work as a team; monsters who feed off each other's warped psyches to commit murders they never would have carried out on their own.