Kim Edwards and Lucas Markham were just 14 when they brutally murdered a mum and sister.
Kim's mother, Elizabeth, 49, and her younger sister, Katie, 13, were murdered in their own home on April 12, 2016.
Markham stabbed Elizabeth in the neck before smothering her with a pillow and then moved on to kill 13-year-old Katie as she slept, claiming he murdered her because he thought she would call police.
Following their chilling crime, the teenagers had sex, a bath and then cuddled together to watch the Twilight film.
The horrific crime in Spalding, Lincs, shocked the nation, as the pair became Britain's youngest convicted double murderers - dubbed the Twilight killers .
Edwards and Markham were described as "inspearable" and Elizabeth was said to be "scared to death" of her daughter's boyfriend.
But what could drive two children to commit such a violent and gruesome crime?
Britain's Deadliest Kids, on QuestRed tomorrow, looks at possible motives for the young couple to carry out the murders.
One of their former friends, Adam Free, reveals how Markham developed an "unhealthy" interest in the murder of his father - and how the killers got away with it.
Warren Free was just 42 when he was viciously attacked outside his own home as he tried to protect it from a gang of teenagers.
He was beaten with a metal pole and died from head injuries in August, 2014.
Warren and his family lived close to Elizabeth and her daughters.
An 18-year-old, two 15-year-old boys, a 17-year-old and two 15-year-old girls were found not guilty of murder of manslaughter following a trial the following year.
The trial heard Warren had confronted the group over damage to his property.
Defence lawyers claimed the teenagers had been acting in self defence.
His son, Adam, said: "Lucas was on my side with my dad's murder but he sort of wanted to know a bit more about what happened and how they got away with it really.
"I guess they were thinking about how they could get away with murder because the people who killed my dad did."
Paul Cheston, a former crime correspondent, added: "Friends of Lucas point out that he was obsessed with a case that had occurred shortly before his own murders.
"Lucas's friends think that he was inspired by this - that he could actually murder and get away with it."
Chillingly, Kim's mum and the pair's first victim, Elizabeth, was terrified of her daughter's boyfriend.
Her partner, Graham Green, said: "She was scared to death of him because he had a bad temper on him."
Devastated Graham also never trusted Markham - and says the pair has destroyed his life.
He added: "Elizabeth was organising for us to get married. We would have had 30 years together.
"Lucas and Kim destroyed my life."
The programme also reveals how Markham had been bullied at school over his mother's death from cancer and that he had been placed in small unit for pupils with behavioural problems in the summer of 2015.
The couple were also described as "creepy" because they were always together.
They are thought to have planned the murders because they believed Elizabeth was trying to split them up.
They even plotted how they would kill Kim's mum and sister in their local McDonald's.
Markham went on to brutally stab to death school dinner lady Elizabeth, also known as Liz, as she lay asleep while Kim watched.
He then stabbed Kim's 13-year-old sister Katie in her bed.
Both were jailed for life and must serve 17 and a half years behind bars.
- Britain's Deadliest Kids is on Quest Ed and the QuestOD app at 10pm on Saturday.
