Johann Hari: The child who kills is the child who never had a chance

Everything we know tells us child killers are invariably victims of extreme abuse

I have met children who became killers several times in my life: in the warzones of the Congo and the Central African Republic, and in the grey Young Offenders' Institutes of Britain. When I read about the events that are alleged to have happened last weekend in South Yorkshire, I kept thinking about their small, paranoid eyes. Two brothers – aged ten and eleven – have been charged with torturing two other, younger kids. The victims had been hit with bricks, burned with cigarettes, and slashed with knives in a wild field.

We are a long way from knowing what happened in that field that afternoon, or who carried out these acts. The visceral temptation when any child faces such accusations is to brand them as inherently evil demons who should be locked up far from us for life. But the most famous case of child-on-child killing in British history – that of Mary Bell – shows us how flawed this initial reaction is.

In 1968, in the sagging streets of the poorest part of Newcastle, a ten-year-old girl strangled two toddlers – Martin Brown, and Brian Howe – to death. She then cut their bodies, and with her best friend, a mentally disabled 13-year-old, she left notes in a nursery saying: "We did murder Martain brown, fuckof you BAstArd." She was reflexively described in the press as a child who had been "born evil", a "monster" and "demon".

Now we know what happened to her to make her into such a child. Mary's mother, Betty Bell, was a severely disturbed alcoholic who had been sectioned at least once. She worked as a prostitute specialising in sado-masochism – whippings and stranglings. The first thing she said when Mary was placed into her arms after giving birth was: "Take the thing away from me!" She rejected her daughter and repeatedly tried to kill her by feeding her an overdose of sleeping tablets. But eventually, she did find a use for Mary. Once she turned four, she began to pimp her to paedophiles.

Mary never knew who her father was, but she suspected her mother had been inseminated by her own dad. Later in life, she asked her mother point blank if this was the case. She didn't deny it. Betty simply said quietly: "You are the devil's spawn."

When she was ten, Mary made friends with another girl who was being raped by a local paedophile. All they had known in their lives was violent abuse – and they began to act it out. Mary tried to cut off one of the boy's penises with a razor – a plain, crazed act of revenge for what she had experienced since she was a toddler.

Yet it is strangely comforting to see evil as a primordial external force, something alien that can be hunted down and confined to cages. It dodges the colder truth that I have learned from all the child-killers I have met: we all have the capacity for terrible cruelty and sadism, especially if we are subjected to horror ourselves. Which of us can be confident that, given such Mary Bell's childhood, we wouldn't have done something depraved?

Yet the trial of the two children who killed Jamie Bulger – and the websites trying to figure out where they are now, so they can be lynched – suggests we have barely progressed since then. Excellent works of investigative journalism like Blake Morrison's book As If have uncovered evidence that these children were subjected to violent and probably sexual abuse. We don't want to hear it. We want devils and demons and a black-and-white world that tells us: no, it couldn't have been you; this crime belongs to a different species.

These killings are not political parables. However much right-wingers want to make this a story about welfare dependency and left-wingers want to make it a story of brutal Thatcherite economics, these rare murders have happened in Britain at the same rate for over a century. They have to be understood at the personal, human level.

To understand and explain these cases is not to excuse, or justify. We are talking about the most terrible thing that can happen to a person: torture, and murder. The children who do this need to be humanely detained for as long as they are a danger. But everything we know about children who kill tells us they are invariably victims of extreme abuse themselves, deserving of compassion, not hysterical condemnation.

I have watched my friend Camilla Batmanghelidjh – the director of Kids Company – work with children in South London who have bricked, bottled and tortured other children. She explains: "Since the Bell and Bulger cases, we've learned a lot about how a developing brain reacts to abuse, but the judicial system hasn't caught up.

"We now know from brain scans that if you have really poor quality care in childhood, your pre-frontal lobes don't develop properly. Those are the parts of the brain that think rationally, empathise, and exercise self-control. It is physically impossible for these children to calm down and think a situation through. Their brains haven't developed that way."

So to treat them like morally responsible mini-adults who just made a bad decision – as the British courts do today – doesn't make sense. It is a neurological fiction.

When this impaired brain chemistry combines with violent abuse and rape, the children can become time-bombs. "They have been taught to see the world through one template: you're a victim, or you're an abuser. That's how they think human relationships work," Batmanghelidjh puts it. "At first, they are abused, and at some point they become determined to be a perpetrator, because then at least they have power and control. If you think those are your only two options in life, it seems preferable."

As she said this, I remembered the child soldiers in Central Africa who pointed guns into my face and smirked. Their families had been bayoneted in front of them, and they had buried the bodies themselves. In the warzones of the Congo, I met 11 and 12-year-old boys who had seen their mothers and sisters snatched away, and were then picked up by the militiamen and trained to rape and kill. Like Mary, they were re-enacting the violence they had experienced in a desperate attempt to switch roles: this time, they were the Big Men.

Children who kill are a question of mental health, not morality. They are internally destroyed children, not devils. Given the love and support that they deserve, such children can develop their frontal lobes and their capacity for empathy over time, and be released. As Gita Sereny's reportorial masterpiece Cries Unheard shows, Mary Bell eventually developed into a morally responsible adult and "a very, very loving mother" – albeit one perpetually haunted by the knowledge of what she had done.

Haven't we progressed enough since the Middle Ages to see these truths, and reject the barbaric theology of "evil" children?

When accusations like this bleed into the news, we need to stand at the front of the looming lynch mob and say: Stop. Think. In 1861, a leader in The Times commented on the trial of two eight-year-old boys in Stockport who had tortured and killed a toddler. It said: "Children of that age cannot be held legally accountable in the same way as adults. It is absurd and monstrous that these two children have been treated like murderers." Isn't it time we progressed to 1862?