Professor Paul E. Mullen has spent years studying the psyche of mass killers and believes there is one key trait that unites them all having interviewed ten notorious individuals View 3 Images Professor Paul E. Mullen has sat across from some of the most feared killers in history (Image: Paul E. Mullen)

A forensic psychologist who has encountered more mass murderers than virtually anyone else in his profession has disclosed the single characteristic they all possessed. ‌

Professor Paul E. Mullen has questioned 10 of history's most infamous lone mass killers, including figures such as Martin Bryant, the Port Arthur gunman who slaughtered 35 people in Tasmania, and Thomas Hamilton, who carried out the Dunblane school massacre in Scotland. ‌

The Bristol-born expert in cr

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