Spalding murders: What drove teenage sweethearts to kill?

The murders of Elizabeth Edwards and her daughter Katie were as shocking in their chilling brutality as they were in their steely execution. The age of their killers makes the case truly exceptional - so what drove two 14-year-old sweethearts to commit double murder?

Over the course of three days a jury of seven men and five women listened as the grisly detail of the murders of Elizabeth Edwards and her 13-year-old daughter were detailed.

Mrs Edwards, 49, was pinned down on her bed and stabbed eight times, including twice in the neck, her blood spattering across the walls. The boy had purposely attacked her in the throat to damage her voice box.

Why? To ensure her daughter was not woken by screams or cries for help.

Girl, 15, guilty of double murder

'I hope they throw away the key'

As the case was opened one juror was clearly horrified by what she heard, her eyes visibly widening as prosecution barrister Peter Joyce QC outlined the events of 13 April.

The public gallery, packed with relatives of the victims, was not immune either.

As the court heard how the killers had taken a bath to wash off the blood before hunkering down in front of the teenage vampire film Twilight, one man glowered at the dock and hissed: "You're sick".

The judge too described the facts of the case as "exceptional in the sense of being rare and possibly unique in recent times, given the relative youth of both defendants".

But the trial of the 14-year-old girl heard it was not mental illness that lay behind the killings.

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."

He told the court it was a Bonnie & Clyde-esque scenario of "us against them".

The court heard the girl first became aware of the boy in 2013, but they did not become a couple until May 2015.

After the killings, the girl told Dr Joseph the relationship meant she "felt happy for once", adding that they had a lot in common.

"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 jury was told things had gone well between the couple for the first few months but between November and March they had been arguing increasingly.

A relative of the girl had previously described the pair as a "time bomb waiting to go off". A time bomb prosecutors say "went off in April".

Despite the growing cracks in their relationship, they spent days together in advance of the killings, going over and over their plan before executing it with brutal effect.

After the horrifying attack on Mrs Edwards the boy, who admitted committing the murders while aged just 14, walked into Katie's room.

With the same knife used to kill her mother, he stabbed her in the neck before smothering her with a pillow while his girlfriend waited in the bathroom.

Katie's body was covered with a sheet because the girl said she "didn't like the smell of blood".

They then took their bath, went downstairs and sat down to watch the film. The girl ate a snack: ice cream and toasted teacakes.

It was chilling details such as these that led prosecutors to describe the killings as "cold, calculated and callous".

But how can a relationship of less than a year between two young teenagers culminate in murder?

Prof Francis Pakes, a criminal psychologist at the University of Portsmouth, said: "Group behaviour differs from individual behaviour in a number of ways.

"One factor can be that responsibility for the groups' actions is not necessarily as clear when more than one individual engages in criminal behaviour.

"Another factor is that of shifting norms and values.

"If there is an intense relationship between two or more impressionable individuals, notions of what is normal and what's right are more easily shifted; there's a bigger chance of small groups developing more extreme ideas.

"It can be a case of people shifting their thinking from regular norms and values or it can be that their values do not shift but they find it easier to justify behaviour that they commit when under the influence of another."

In some cases, he said, simply talking about death and killing could "heighten a relationship". A morbid fascination with the darker side of life can also be a trait in some young teenagers, he added.

However, he said that talk rarely turns into fatal action.

"You are talking about highly secretive, highly forbidden, highly taboo subjects, so it's conceivable that the very talk strengthens the relationship," he said.

"But, for many people, just musing about a crime is where it stops.

"Most people have an inner emergency brake, but the impression here is that that emergency brake was never applied."

Although the teenagers were a couple, the details of the case do not match other high-profile cases of killer couples, such as Fred and Rosemary West or Moors murderers Ian Brady and Myra Hindley, according to Prof Pakes.

"When you think of these cases, the explanation is often a very generic one where the female is under the spell of the man who is seen as the instigator," he said.

"But there's an extent to which people are unique and it means you can't always reduce their behaviour to a typical case of X or a typical case of Y.

"There are occasions when you simply have to accept these acts on their own terms."