Why did 32-year-old Lindsay Haugen strangle and kill her 25-year-old lover Robert Mast in a Wal-Mart parking lot in Billings, Montana, on the 15th of September 2015? Revenge? Financial gain? Vicious compulsion? Mercy? Curiosity…? Well, it all depends on who you ask.

At the age of 17 years old, Leo Little kidnapped, robbed and murdered Jehovah’s Witness minister Christopher Chavez.

After slashing the throat of 72-year-old Ruth Predmore when he went to rob her home to pay for his sister’s medical bills, Joseph Murphy spent 24 years on death row before finally having his sentence commuted by the then Governor of Ohio, John Kasich a week ahead of his scheduled execution.

On October 13th 1984, after 15 years in an unhappy marriage Linda was planning to demand a divorce and according to Linda Couch that night a fight broke out and a gun discharged accidently, shooting Walter in the back of the head, killing him.

In 1996, David Barnett murdered his adoptive grandparents, stabbing them over 20 times with five different knives. His motivation for doing so remained a mystery until it was discovered that David Barnett claimed to have been subjected to years of physical and sexual abuse, not by his grandparents but by their son, his adoptive father, John Barnett.

Cavona Flenoy was convicted of the murder of a Sudanese student, with prosecutors arguing that she had lured him in with a promise of sex before shooting him and stealing his wallet and car.

Charles Armentrout attempted to murder his father, and successfully murdered his grandmother.

In 1984, alongside two female accomplices, Toby Lynn Williams robbed and kidnapped at gunpoint his former employer, Johnny Moore, his wife, Deborah, and six-month-old baby.

In December 1996, Mark Sam Arthur discovered his friend’s mother – Carmen Fonseca – badly beaten, at the hands of her husband. Whilst comforting the abused woman, Mark says he resolved to avenge her should he ever get the chance.