It was a case that shocked Britain – a young teenage couple, aged just 14, who brutally slaughtered a mum and sister.

And following their chilling crime, Kim Edwards and Lucas Markham had sex, a bath and then cuddled together to watch the Twilight film

But what drove the young teeangers to kill Kim's mother, Elizabeth, 49, and her younger sister Katie, 13?

Dubbed the 'Twilight killers', Edwards and Markham butchered Elizabeth and Katie at their home in Spalding, Lincs, 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.

The pair – who were compared in court to Bonnie and Clyde – had planned the killings together and are the country's youngest double murderers.

Both were found guilty of murder and sentenced to life in prison.

Psychologist Dr Keri Nixon believes she has the answer as she lifts the lid on children who kill.

Dr Nixon, who will appear in new QuestRed series Britain's Deadliest Kids, 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.

"Edwards had a difficult relationship with her mother and was quite jealous of her sister.

"But would the murders have occurred if those two hadn't been together? Probably not.

"Markham felt needed and wanted by Edwards and that became intoxicating. He thought he was protecting her."

Dr Nixon believes childhood trauma plays a huge part in why children kill.

She said: "The majority of the offenders I have worked with, there is something in their background which can help me understand why they have the issues they have.

"It never justifies why somebody would take someone else's life, but it certainly helps me understand.

"We know trauma causes changes to the brain. And we know the toddler years and the teenage years are the two biggest growth spurts for the brain.

"A child who has been traumatised doesn't react in the same way as a child who hasn't.

"Their brain doesn't work the same way as it does in the rest of the population.

"They're going to have behavioural difficulties, personality disorders and won't feel empathy or remorse like the average person."

Lorraine Thorpe was just 15 when she murdered her father Desmond Thorpe and a woman called Rosalyn Hunt in Ipswich in 2009.

She had become used to violence after living on the streets with her alcoholic father and constantly witnessing violent behaviour from the group of addicts they hung around with.

Thorpe became attached to the ringleader of the group, a 41-year-old man called Paul Clarke, who influenced her to commit murder.

Together they had repeatedly beaten and tortured Rosalyn Hunt and then smothered Desmond Thorpe to death to stop him going to the police.

Both were jailed for life and Clarke died in prison in 2014.

But Dr Nixon believes the murders could have been averted if Thorpe's parents hadn't failed her.

She said: "She should never have been living on the streets and socialising with alcoholics.

"She should have been at home and looked after by her parents. They failed her.

"It was a very chaotic lifestyle and she didn't have the usual important boundaries children have.

"There were so many missed opportunities to help a young girl who should never have been in that situation.

But she goes on to be so horrifically violent that it's difficult to feel empathy for her.

"Violence was a part of her everyday life and she was completely desensitised to it.

People talk about her being very manipulative and lying and she was but again she had to be like that to survive on the streets.

"She took it to the extreme, but would she have committed those murders if she hadn't been living on the streets? If she hadn't met Paul Clarke? Probably not."

But Dr Nixon does chillingly admit there have been cases where she has not been able to find an explanation for a person's violent actions.

She said: "It's very rare but there have been times in my career where I've met someone with very little in the background to cause such behaviour.

"You can't find an explanation and you do think this person just seems to be evil. Those cases are quite chilling."

The case of 15-year-old William Cornick is one such example where nothing indicated he would kill teacher Ann as she taught Spanish at Corpus Christi Catholic College in Leeds.

He was academically gifted and had taken his GCSEs early but developed a hatred of Ann and bore a deep-seated grudge against her.

He offered pals "a tenner" to kill her four months before she died, but they just assumed he was joking.

On the day of the murder, he brought a kitchen knife into school and even showed it to classmates, who again wrongly believed he wouldn't go through with his threats.

Instead, halfway through a Spanish lesson on April 28, 2014, he stabbed Ann seven times in the back and neck with his 21cm blade.

Cornick pleaded guilty to the murder, and therefore wasn't questioned. He was sentenced to life behind bars, serving a minimum of 20 years.

Dr Nixon adds: "There were none of the usual disturbing signs. It could have been anyone's child.

"We didn't see the usual background in the case that we would usually see in someone who would be very violent.

"That was what was so shocking. It was also a case of bystander apathy, his friends assumed somebody else must be dealing with it because what he was saying was so bad.

"But when we see or hear behaviour like this, we need to be aware and report it."

• Britain's Deadliest Kids premieres at 10pm this Saturday exclusively on Quest Red.

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