Aged just 14 Kim Edwards and Lucas Markham became the youngest double murderers in the UK.
The teenage couple killed Edwards' mum, Elizabeth, and her younger sister, Katie, and were compared to Bonnie and Clyde during their murder trial.
Markham stabbed 49-year-old Elizabeth in the neck before smothering her with a pillow.
He then killed a sleeping Katie, claiming he had no choice but to end the 13-year-old's life because he feared she would tell police what he had done.
Disturbing audio of Edwards' police interview reveals how she heard her mum ask for help as Markham murdered her.
She said: "The fact that it happened so quickly that like gave me peace of mind cos like, you know, it wasn’t like torture or anything so. (sic)."
She also said her 13-year-old sister's death meant she didn't have to go through "heartbreak" and "emotions".
This wasn't a sudden crime of passion, Markham and Edwards had been planning the murders for some time.
And they had slashed their victims' throats to stop them from screaming.
Markham and Edwards believed Elizabeth was trying to split them up and even worked on their sinister plan in the local McDonald's.
Terrifyingly, Elizabeth had been scared of her daughter's boyfriend before the night he stabbed her to death.
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."
After Markham and Edwards, from Spalding, Lincs, had carried out the murders their actions were truly chilling.
The teenagers had sex, then shared a bath before cuddling up on the sofa to watch the Twilight film, which earned them their horrifying name, the Twilight killers.
They were both found guilty of murder and jailed for life.
Markham and Edwards were described as "inseperable" by those who knew them, with some classmates found them "creepy" because they were always together.
And one of the pair's former friends, Adam Free, has opened up about Markham's "unhealthy" interest in the murder of his father.
Adam's dad, Warren, was just 42 when he was viciously attacked outside his own home as he tried to protect it from a gang of teenagers.
He had confronted the group over a damage over damage to his property and was beaten with a metal pole.
He died from head injuries in August, 2014.
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."
Psychologist Dr Keri Nixon believes Markham's troubled background could have sown the seeds of his murderous behaviour.
She said: "Markham had a difficult background, he had multiple placements in care which we know is a really big risk factor.
"His mum died, his dad was an alcoholic and he witnessed domestic abuse.
"There were some IQ concerns, there was truanting and trouble at school, he had some mental health issues and trouble regulating his emotions.
"He was a very damaged young man."
Meanwhile, Dr Nixon, says Edwards had a difficult relationship with her mum and was jealous of her sister.
But she says it's unlikely the pair would have carried out the killings if they hadn't been together.
Dr Nixon said: "Markham felt needed and wanted by Edwards and that became intoxicating. He thought he was protecting her."