Robert Thompson and Jon Venables, aged 11 and 10 at the time, were found guilty in 1993 of killing 2-year-old James Bulger. Photo / Getty

"I want to kill all of you."

That was the terrifying threat uttered by 6-year-old Samantha*, leaving her parents shocked and in a state of unimaginable fear.

After being caught trying to strangle her sister in the back seat of the family's car, Samantha - adopted in to the family - was sat down for a stern talking to. Her mother explained to her that she could've killed her sister.

"I know," Samantha said blankly. Then she told them that was her plan. She wanted to kill them all.

Samantha had grown up with a desire to inflict pain on others, according to reporters who spoke to the family as part of a story featured in The Atlantic this week.

She practised killing by using her toys and drew disturbing pictures depicting murder weapons, including a plastic bag for suffocating and chemicals for poisoning.

She urinated on a child at daycare even though she was toilet trained and would break open her sister's piggy bank and tear up her money.

She would also smile when her siblings cried and push and pinch them. She didn't outgrow the behaviour.

Now 11, Samantha spends her time in a treatment facility south of Austin, Texas, trying to control her violent behaviour. She's been diagnosed with what experts call "conduct disorder with callous or unemotional traits".

"She had all the characteristics of a budding psychopath," The Atlantic reports.

There are a number of chilling stories of children who have acted on dangerous impulses.

One of the most horrific cases of child killers involved 2-year-old James Bulger, who was savagely murdered by two boys aged just 11 and 10.

James was in a British shopping centre when two boys, Robert Thompson and Jon Venables, grabbed the trusting 2-year-old and led him away.

They sadistically tortured and beat James to death and rested his broken body on train tracks.

The boys were found guilty of abduction and murder in 1993 and were jailed. Venables was released in 2001 after serving eight years. Thompson was protected by draconian rules and it is believed he has a new identity.

THE MAKINGS OF A PSYCHOPATH

Senior clinical psychologist and director at the R.E.A.D Clinic, Heather Irvine, told news.com.au the first year of a child's life plays a significant role in determining how they develop.

"Psychopaths don't suddenly come out of nowhere, you need to look at their early history and what's gone on with that child from when they've been born," she said.

According to Irvine, most children who have psychopathic traits have experienced some sort of abuse, likely in the first six months of their life.

"In terms of how a brain develops, if they've been abused, the emotional system starts to shut down as the rest of the brain is developing. The brain ends up being incredibly impacted because of experiences with abuse, neglect or lack of empathy."

Irvine said if a child had grown up in a nurturing environment but then began suffering from abuse after they turned 5, it would be much harder for that child to develop sociopathic or psychopathic traits because the brain had already developed a sense of empathy.

"In the first six months of your life if you cried and nobody came, you needed to be fed and nobody fed you, you were alone and nobody helped you, your brain is saying your feelings don't matter, you don't matter. The rest of the brain structures start forming around that concept and neurons develop around that concept," she said.

TRAITS 'VERY UNCOMMON'

Irvine said callous and unemotional traits in children was very uncommon, with it affecting only one per cent of the population.

"The point is that everything's on a scale, it's not really a case of really good kids and really bad kids," she said.

"The more severe the abuse, the more likely a child is to have severe psychopathic or sociopathic behaviours."

Irvine said there were specific behavioural issues children with psychopathic traits had.

"Generally tends to involve damaged property, damage to others or damage to self," she said.

"You see kids starting to harm animals, punch holes in walls, urinate over furniture or smear poo over toilets. They will inflict significant harm on another human being, usually smaller than them.

"They are trying to understand connections, usually through harm rather than care."

DO PSYCHOPATHIC CHILDREN BECOME KILLERS?

Irvine told news.com.au a child's brain could start developing callous and unemotional traits from birth. While many psychologists work to help children control their violent tendencies, some can grow up to make poor life choices. Others grow up to be killers.

"You always have hope a child won't grow up to be a killer, but without significant intervention we would predict poor outcomes," Irvine said.

"They find it hard to succeed in school or make friends and are more likely to engage in a whole range of anti-social behaviours to gain attention or find some kind of place in this world.

"They're therefore likely to engage in substance abuse and spiral downhill unless there's significant intervention."

Irvine said if there were any concerns a child wanted to inflict harm on another person, on animals, or damage property, they needed to see a GP, paediatrician or psychologist for early intervention.

"A child will hit, poke or punch but that doesn't indicate a sociopath. It's really important parents understand persistent behaviours that show a complete disregard for other people's feelings," she said.

Irvine said continuously inflicting pain on others, disregarding property, and being unable to repair relationships were also causes for concern.

Irvine said parents needed show their children repetitive love and care to rewire the brain.

"Parents need to be prepared to do the hard work. Children don't need therapists and hours of therapy, they need a loving home," she said.

* Not her real name