They planned it for days. They would prefer to do it with a gun but they couldn’t work out how to access one so they decided to use a knife instead.
Together they sat in McDonalds and discussed how they would murder the woman who ran classes at the local church and worked as the school dinner lady, along with her 13-year-old daughter.
The girl, who was 14 at the time was the “driver” of the plot, the court heard she had a “grudge” against her intended victim.
She persuaded her 14-year-old boyfriend to take part, telling police: “I’ve felt like murdering for quite a while and [the boy] just hates me being upset…it just sort of happened.”
On the night of the murders in April 2016, the pair entered the home of Elizabeth Edwards, 49, and Katie, 13, in Spalding in the UK, bringing with them a bag the boy had with four knives.
The boy then straddled Mrs Edwards as she slept, pinning her to her bed before repeatedly stabbing her in the throat and voice box to prevent her from crying out.
The girl told police she heard Mrs Edwards cry out “get off me”.
“But I’m not entirely sure because she was gurgling.
“I saw her struggling with her arms. I could hear her legs kicking out and crashing onto the chair.
“He has claw marks on his back because she clawed him to get him off her.”The girl said at one point she thought she saw the boy’s hand and reached out to it but it turned out to be Mrs Edwards’.
“I touched the hand and realised it wasn’t his.”
He then carried out a similar attack on her 13-year-old daughter. Smothering her and stabbing her.
The court heard that Katie was wrapped in a sheet by the duo because the girl “did not like the smell of blood” that had splattered the walls of the bedroom.
They then took a bath together and had sex writes The Lincolnshire Reporter.
In her police interview the girl said: “I planned it so we would be clean and refreshed and wouldn’t smell of blood. We spent about 20 minutes in the bath.
“I like Twilight, so I said to [the boy] ‘Why don’t we watch it?’ I thought he might like it as well.”
The court heard that the two had talked about taking an overdose together after the murders, but decided they didn’t want to so instead they ate ice-cream and toasted tea-cakes instead.
In the months leading up to the killings the girl wrote in her diary, “there is madness in me” and “death is the only way”.
She was described as a “ticking time bomb” who was fuelled by “sheer contempt and brutality” for her victims.
The boy earlier pleaded guilty to the murders, but the young girl had denied murder, admitting only manslaughter.
However yesterday the court rejected this, finding her guilty of murder too.
The trial had heard that it was not mental illness that lay behind the killings but a toxic relationship.
Expert witness Dr Philip Joseph, a consultant forensic psychiatrist, said she was not suffering from a mental disorder as she had claimed and the key to the murders was the relationship between the young couple.
"If they had not had this intense, toxic relationship this would never have happened. It is the relationship that is behind the killings," he said.
"When you have two people together like that, the group dynamic can lead on to a course of action that would otherwise never have taken place."
“Bonnie and Clyde...that sort of intense attraction, emotional closeness - them against the world. It's that sort of thing that led on to this.”
During her trial it was revealed that the girl told Dr Jospeh the relationship meant she "felt happy for once.”
"He was my first serious boyfriend. We started having a sexual relationship. It was the first time for both of us," she said in interviews.
"I felt very close to him. It was the closest I've ever felt to anyone.
"Nobody else liked him because he was annoying. Until he came along no-one ever listened to me."
The pair will be sentenced in November.