Yet aged just 15, Farhad Khalil Mohammad Jabar was not too young to find a gun, and not too young to murder a man, a stranger who committed no error but to leave work for home and family.

The cause of the widespread shock since Friday was not the fact alone of a cold execution for an obscure political purpose at the headquarters of NSW Police. It was at the age of the executioner.

He should have been enjoying his school holidays worrying about teenage dreams, rather than ending the life of a husband and father before the reward of his retirement.

It is difficult to fathom how anyone could be so ruthless. But a 15-year-old? That takes some fathoming. How was someone ineligible for a driver's licence able to find a revolver? As a neighbour asked a reporter this weekend: how does a kid get a gun?

This is not the first time the age of a murderer has shocked us. Matthew Milat, great-nephew of Ivan, who changed his surname to Milat after his uncle's mass-murdering was exposed, was only 17 when he tortured and murdered a teenager who thought they were friends.

Britons Robert Thompson and Jon Venables were both just 10 when they murdered toddler James Bulger and shocked not just their country, but the civilised world.

Nor is it the first time a teenager was linked to terrorism; last year in Melbourne, 18-year-old Numan Haider was shot dead outside Endeavour Hills police station after he stabbed two AFP officers. At least he was an adult, if barely. Farhad was just 15.

Yet child offenders tend to commit crimes against property rather than people when compared with adults, which is in part why Farhad becoming a killer is so shocking: it's so rare.

We may discover why he killed, but we may not. It is possible the only person who knew why a child took an innocent life lies in the morgue.

What we can do is redouble our efforts to divert children who offend, or are heading that way, from taking the destructive criminal path into adulthood, and not just those with links to terrorist groups. There will be much said, properly, about deradicalisation, but the task goes well beyond the tiny subset of crime which can be called terrorism. We can do  much more for a safer community by helping those most at risk of committing crime after growing up.

The biggest risks for child offending are family violence and unemployment, abuse and neglect, mental ill health and intellectual disabilities and, abominably, the fact of being removed from family dangers and put into care.

Children are more vulnerable to perversion, which is presumably why terrorist groups put so much effort into recruiting them. But when they offend, they are also particularly deserving of efforts at rehabilitation.

Child offenders are often obvious products of neglect, but even when not, young brains do not finish developing until well into the 20s. As a consequence adolescents tend to take more risks than adults and are more susceptible to peer-pressure.

And even without knowing the motive and influences behind the Parramatta killing, surely the message of intervention and help is one way to find anything positive to come from this.