In 2004 I met a man called Jack who had killed his mother when he was in his 20s. He had been found to be suffering with paranoid schizophrenia at the time, so he was sent to hospital for treatment.

Later, he joined a therapy group that I was running at Broadmoor Hospital. In the hour-long sessions the group members, who had all killed family members while mentally unwell, would talk about how they could avoid violence in future. Jack didn’t always seem engaged but after a year or so, just after another member had talked about past regrets, he spoke abruptly.

“I wish I could say sorry to my mum for what I did,” he said. “I know I was mentally ill, but I wish I could say how sorry I was and that she could forgive me. I hope she understands how much I regret it.”

By seeing themselves in other offenders, some group members were able to learn how it had been possible to delude themselves into thinking that someone had to die; and how waves of anger, shame and fear could lead them to misinterpret actions and words.

Jack seemed more engaged after that day and his mental health improved enough for him to move to a less secure hospital for further rehabilitation.

Group therapy takes time, but afterwards many other men were also considered safe enough to move to less secure treatment facilities, which is a sign of improvement and something we only do if we determine that their risk of reoffending is negligible. Most importantly, they also learnt to take responsibility.

Jack helped me realise that people who kill are not mindless monsters who are born that way. He was an ordinary man who had done an extraordinary thing, as with many others.

None of this is an excuse for violence – and every violent crime is a tragedy for all who are involved – but monstering people is not helpful. It is simply one way to deal with rage and fear. And we miss a chance to reduce and prevent violence if we write off everyone who has murdered or abused in that way.

It takes a radical kind of empathy to sit with a man who has decapitated his partner, or a woman who has stabbed a friend. But trying to comprehend them and gain new insights about ourselves requires going where they walk, and seeing what they see. And that is what ultimately leads to change.