Tim Watson-Munro knows what it means to stare into the eyes of evil.

For four decades, the forensic psychologist has been analysing the minds of some of the worst criminals of our times: terrorists, mass murderers, sex offenders, torturers of children.

"A lot of the criminals I've seen, particularly at the violent end of the spectrum, it's constant urban warfare for them," he says.

"They are psychopaths, they have no empathy, they kill at random or for money."

Watson-Munro has worked with about 20,000 people, but a guy by the name of Johnny Cribb sticks out in his mind as the worst.

"He was a prisoner in Parramatta Jail where I started 40 years ago. He had abducted a mother in the Hills District of Sydney, with her three children," Watson-Munro says.

"He kept them captive for three days and ended up killing the lot of them.

"He is the closest I've come, I believe, to evil simpliciter."

Sorry, this audio has expired Tim Watson-Munro talks to Hugh Riminton about his experiences - both professional and personal

Watson-Munro's job, simply put, is to work out whether an inmate is legally insane.

And while it's easy to assume the people who commit acts of extreme violence and depravity must be crazy, Watson-Munro says the "definition of legal insanity is much higher than that".

He cites the case of serial killer Peter Dupas, who murdered three women and is suspected in the deaths of three others.

"He has killed a lot of women. He is an abomination, and in my view he should never be released," Watson-Munro says.

"But he's a classic example of a person who is very bad, not mad.

"He doesn't have a disease of the mind beyond the fact that he is a psychopath."

Mass murderer and 'bewildered kid'

Watson-Munro has also dealt with Hoddle Street killer Julian Knight, who shot dead seven people and injured 19 others in 1987.

Knight is one of Australia's most notorious mass murderers — but Watson-Munro doesn't believe he is all evil.

"I don't think he's the most evil, dangerous person I've encountered, not by a country mile," he says.

When he first went to meet Knight in prison, he didn't know what to expect.

"I didn't know whether I'd encounter an enraged psychopath, somebody who would refuse to see me, someone who was acting out," Watson-Munro recalls.

"And much to the contrary, I found this bewildered 19-year-old kid."

Through his career, Watson-Munro has learned there are many shades of grey in the human condition.

Knight had grown up in a military home, had a history of "being bullied and bastardised", and was allegedly abused during his time at Duntroon military college.

Over many years the teenager's anger and rage festered — culminating in a devastating act.

Knight killed seven people and injured 19 in less than an hour. ( ABC News )

"The critical aspect to this, beyond obviously his psychopathology at the time, was that he had three high-powered weapons in the home and rounds of ammunition," Watson-Munro says.

The psychologist is a "very firm advocate" for gun control, and says you only need to look at the situation in America to see that the law can prevent mass shootings.

"You contrast our position in Australia with the US where it's almost a daily occurrence now," he says.

"You've got disaffected young angry teenagers bullied at school, a similar theme, bullying, being persecuted, and they go down to the gun store and they come back and wipe out a lot of kids. It's terrible."

Rising, falling, and rising again

Being exposed to the worst aspects of humankind took its toll on Watson-Munro.

In the late '90s, his life came crashing down.

"I developed a severe depressive disorder. It was the high-flying '90s. I had a very high profile, and I developed a very severe addiction to cocaine," he says.

"It was intense, short-lived, it was publicly exposed. There was a huge amount of opprobrium, justifiably — I've let a lot of people down with that.

"I lost my right to practice for nearly four years."

Watson-Munro documented his highly-publicised fall from grace in his searing memoir, Dancing with Demons.

"I worked on my recovery, and it's been onwards and upwards ever since that time," he says.

He has now released a second book, A Shrink in the Clink, which further explores his career — including the unique psychological threats faced by first responders.

"I'm very sympathetic to police, ambulance officers, fire brigade people, nurses — anyone who deals with crime at the coalface," he says.

"Like me, I think they develop symptoms of vicarious post-traumatic stress because of the nature of the work they do.

"I see people privately, they are struggling with their demons.

"And I think there needs to be more of an open conversation about that."