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A forensic psychologist who has spent more time with mass killers than almost anyone else in his field has disclosed a common trait they all possess.

Professor Paul E. Mullen has come face-to-face with 10 of the worst mass killers in history, including individuals such as Martin Bryant, the Port Arthur killer who took the lives of 35 people in Tasmania, and Thomas Hamilton, the man behind the Dunblane school massacre in Scotland.

Born in Bristol, the expert in criminal psychology doesn't view these killers as "incomprehensible monsters", but rather as deeply troubled individuals shaped by identifiable forces. With this perspective, this reporter asked Paul if there was a similar characteristic among the lone killers he had interviewed.

He confirmed there was, reveal

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