This opening episode of new true crime series Encounters With Evil delves deep into the dark world of those that kill, not for revenge or in a fit of rage - but just for the sheer thrill of it. British serial killer Denis Nilsen - who murdered at least fifteen people - took sexual pleasure from killing and kept the corpses in his home for as long as possible as a twisted form of 'company'.

A look at three cases of psycho killers, incapable of empathy or remorse. These include Joanna Dennehy, who murdered three men in a ten day killing spree.

This episode investigates four cases in which girls and young women are held against their will, often for many years. We explore the brutality, cruelty and control imposed on their victims by some of the world's most twisted jailers, including: Wolfgang Priklopil, who kidnapped 10-year-old Natascha Kampusch, the notorious 'Black Panther' Donald Neilson who killed heiress Lesley Whittle, the monstrous Josef Fritzl who kept his own daughter in an underground dungeon for 24 years, fathering seven children with her, and sexual sadist Ariel Castro who imprisoned three girls as his sex slaves for 10 years.

This episode of Encounters with Evil exposes the twisted Cult leaders and false prophets who, through a mix of fear and brain washing, led their followers to horrific ends. From the needless and bloody show down at Waco where the followers of the self proclaimed, "Son of God" David Koresh died in an apocalypse of their leaders making.

Cases of kidnappers who kill their victims are examined.

The stories of individuals who do the unthinkable and kill those closest to them - family killers. Starting with Michell Quy, a newly married man who, when his wife goes missing, spins a web of lies in the media to deflect the police from the grisly truth. Jeremy Bamber, the unlikely killer who murdered three generations of his family. And finally, Stuart Hazel, convicted of abusing and killing his partner's granddaughter and hiding her body in their attic.

Cases of cannibal killers included Jeffrey Dahmer are examined.

Cases of Spree Killers are examined.

Cases of sex crimes are examined.

Cases of nurses and physicians who kill are examined.