'I've met 10 of the worst mass killers in history and they all shared one chilling trait'

A forensic psychologist who has spent time with more mass killers than nearly anyone in his field has revealed the one trait they all shared.

Professor Paul E. Mullen has interviewed 10 of the most notorious lone mass killers in history, including the likes of Martin Bryant, the Port Arthur killer who murdered 35 people in Tasmania, and Thomas Hamilton, the man responsible for the Dunblane school massacre in Scotland.

The expert in criminal psychology, who was born in Bristol, believes the killers are not "incomprehensible monsters”, but rather deeply disturbed people shaped by knowable forces. And with that in mind, we asked Paul whether the lone killers he had interviewed all shared a similar trait.

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Confirming they did, he revealed to the Mirror: “A common psychological feature in these people is intense resentment directed at the world in general.

“Grievance is anger directed at specific experiences and perceived injustices or humiliations. Resentment is the constant reliving of grievances in which the individual failed to respond usually for fear of the consequences.

“Eventually resentment becomes central to how the individual experiences the world.”

Paul said most mass killers are isolated with few friends and they consider themselves to be victims whose failure in life is to be explained by the “malevolence” of others.

He added that many are obsessed with guns and that one mass killer he assessed had moved from China to Australia just so he could join gun clubs.

Giving further insight, he said abuse as children was rarely a factor, explaining: “They tend to be white males from middle class families.

“Gross abuse and neglect rarely has prominence in their history as children. Most are of normal or even above normal intellectual ability. In short, they are very different from the vast majority of violent offenders.

“A high proportion have extreme right wing political views together with racist ideas. A small number subscribe to fundamentalist religious beliefs.”

Paul has just released his latest book, called Running Amok, which delves inside the minds of mass murderers. He examines the forces that unite them while offering guidance on recognising warning signs and improving threat assessment.

And while promoting his new book, we asked him if he was ever fearful sitting across from men who had committed unthinkable atrocities.

Setting the scene of what those memorable encounters are actually like, the professor said: “When you are sitting alone in a room with someone who has committed terrible acts of violence they no longer have a gun in their hand, or a knife, or a bludgeon, nor are they reaching for your throat.

“You are facing a frightened individual, who may try to hide their fear by bluster, but for whom you may be a help but are unlikely to be harmful.”

Paul's new book is available to buy here