Cho Seung Hui, the Virginia Tech gunman, may have taken to the grave what, in the end, pushed him over the edge to murder 32 people. But as a psychological profile of a mass killer he was, in many ways, the walking blueprint.

After the Columbine High School massacre in 1999 experts drew up a checklist of warning signs that might alert teachers and peers to worrying behaviour in a student. They included obsession with violence, resentment, a need to blame others, arrogance, contempt and unusual isolation.

Cho, a “weirdo loner” who had been treated for depression, wrote copiously about suicide and death and had recently tried to start a fire in a campus dormitory, ticks virtually every box. The 23-year-old son of South Korean immigrants who had moved to the US when he was 8, rarely removed his sunglasses and hat, presenting a barrier to the world. He took pictures of his classmates in lectures but never spoke. He had so unnerved his tutors that when the head of English agreed to see him she was asked if she wanted protection. She declined, saying that she thought Cho “exuded loneliness”. In the month before the killings he had stopped attending class.

And there was an extra aggravating feature.

Psychologists agree that people driven to such extreme behaviour have usually suffered some form of rejection, loss or affront. Crucially, they believe that their hurt feelings have not been acknowledged, their distress not taken seriously. So their frustration at the external world turns to seething rage which, finally, they can vent only by killing — often in a highly public way. Cho was believed to be infatuated with a beautiful student, Emily Hilscher, 18. His feelings were evidently not reciprocated since she wrote enthusiastically on her website page of a “wonderful” new boyfriend. She was among the first two people to be killed by Cho. It was possibly a misguided sense of rejection and a mad fit of jealousy which, in his fragile mental state, was the final straw for him.

But most peoplesuffer rejection and loss at some stage in their lives and do not turn into mass murderers. Why does it tip some people — almost always men — over the edge but not others? The Times psychologist Dr Tanya Byron said that we all have underlying predispositions to certain emotional and behavioural traits. One person might be more predisposed to, say, depression, another to alcoholism partly through genetics, partly through how they are socialised. But a combination of factors including stress and the environment can combine to push some people into more extreme behaviour. This is shown by the 1977 Stress Vulnerability Model, which proposes that people with a vulnerability to psychosis are at a proportionate risk of developing a psychosis as their stress levels increase.

“The vulnerable person might never have behaved that way if they had a strong family or very good friends around them, for instance,” she said.

“But if the factors aren’t mitigated against and that vulnerability is accelerated, disastrous things can happen.” She added that such killers usually lack social skills and are withdrawn. For them huge feelings of inferiority can be overcome by the use of a weapon. The profile of such young men usually showed them to be overly anxious, often having had their hearts broken. “They don’t know any other way to express their rage,” she said. Dr Byron also believes that a significant contributing factor is a culture which “normalises” tough-guy gun killings such as in blockbuster films and computer games, blurring the boundary between fantasy and reality. “There is a macho gun culture in Hollywood — the hero in the blood-soaked vest taking everybody down,” she said.

“It desensitises a lot of young people to violence. For the very vulnerable it gives them a platform, a licence to express their extreme and deviant behaviour. Anyone who says that the media doesn’t play a part in this is being naive.

“This is a public health issue. We need to ask: how has someone with a vulnerability been moved to committing such acts of violence? What were the triggers?”

It is a fact that in the vast majority of cases such killers are men. This is because females tend to direct anger inwards, via self-harm or eating disorders, while males direct it outwards. Dr Jack Boyle, a child psychologist, said that gunmen like Cho are totally distinct from serial killers who go to great lengths not to be caught. This type of killer orchestrates a spectacular suicide in a public place, often after murdering the person whom they feel has rejected them.

Dorothy Rowe, a psychologist and author, said that school and college campuses may exacerbate extreme behaviour because they are enclosed, separate worlds. “Things that may be quite trivial take on an intensity because you cannot get away from them,” she said. “All you see are other students. It is like a hothouse.” Dr Rowe said that such people need to be listened to, their fears heard. A dangerous person becomes even more dangerous when they are alienated. If they feel their anger is unacknowledged, said Dr Rowe, they will, in rare cases, go to the extreme of wickedness to be noticed. And America is a difficult place for outsiders.

“In the US there is a lot of emphasis on conformity,” she said. “Children and young people who find it hard to conform have a difficult time.” Indeed, the two Columbine killers were known misfits who wore trenchcoats and had suffered from bullying.

The “disconnected” loner might in the end give up and go to the other extreme. “It’s as if they say: ‘If people won’t acknowledge me for my goodness, I will force them to acknowledge me for my wickedness’,” said Dr Rowe.

But she added that no psychologist would accept that there was such a thing as inborn wickedness. “We merely get trapped by our own ideas and our ideas grow out of our experiences. The process of therapy was to help the person see how they had trapped themselves.

“To ask what are we going to do about the guns (law) is futile. The guns come at the very end,” she said. “It’s nota question of restricting guns — it is asking what happens to young people who grow up in a society that doesn’t meet its very ordinary needs.”

Tragically, there seems little doubt that such a massacre will happen again. Copycat killings are a major concern in America. Dr Herbert Mandell, a psychiatrist specialising in gun crime, said that the Virginia gunman had broken the record for the number of students killed. “Someone is going to try to top that,” he said. “If someone is on the edge this might be enough to push them.”