In the small town of Aberdeen, Australia, everyone thought Katherine Knight a little crazy. Suffering from a psychological disorder, her moods oscillated wildly between love and violent anger.

Working at the local meatworks, the woman was obsessed with knives and had the skill to use them. But nothing would prepare the town - and the nation - for Katherine Knight's ultimate act of revenge.

The brutal slaying of Knight's defacto partner in February 2000 would have far-reaching effects. After investigating the case Detective Sergeant Bob Wells would later be forced to take a desk job, traumatised from what he witnessed at the crime scene.

Journalist Peter Lalor, at first restrained from reporting the horrific facts to the public, would eventually pen a book on the killer. David Kellett, Knight's first husband, now fears for his life. He's been warned that he'll be Knight's next victim if she's ever released from prison.

From an undisclosed location, David Kellett tells his extraordinary story for the first time on television. With Detective Wells and Peter Lalor, all three men share insights to a crime that is unparalleled in its shocking detail.

If there is any solace for Katherine Knight's victim, it's that he only died once. For Raymond Reid, in North Carolina, death would not bring relief.

Blanche Taylor-Moore poisoned her lover right under the noses of doctors in the hospital. Every time Reid was revived, Blanche would be there to poison him again.

Reid would die a final time in 1986, but it would be years before forensic science would unveil Blanche's venom.

Ruth Ellis was young, attractive and had film star allure. Some maintain that she is a calculating murderer. To others, she is a martyr. New Scotland Yard still holds the actual weapon that Ellis used top slay her boyfriend, David Blakey.

In 1955 Ellis waited outside a north London pub and then plugged Blakey with five bullets. It was an act of revenge that culminated in years of physical and emotional abuse. Nowadays it's unlikely the woman would be found guilty of murder. But courts in 1955 disallowed the defence of 'diminished responsibility' - whether she was justified to pull the trigger or not, Ruth Ellis will forever be remembered as the last woman to hang by British justice.

The city of Vienna, Austria, was outraged by a spate of barbaric murders in the 1980's. They occurred not in the backstreets, but in a place of healing, inside the walls of one of Vienna's largest hospitals. Waltraud Wagner and her nursing colleagues were overworked and frustrated with a thinly-resourced health system.

To ease their workload the nurses revenged patients who demanded too much of their time. The victims were all elderly, many already close to death. No one suspected anything when patients on Ward Five began to expire. Helping to evade detection, the nurses employed a cunning method of execution they called 'The Water Treatment'. To investigators, it would be a forensic first.

Whether unleashed in a single crime of passion, or leaked through the cracks of smouldering resentment, revenge is the dark-side of human emotion. It is the primitive force that turns scornful females into deadly women.