Among the lists recommending the best of this or that out on the net, there are sites listing feature movies about mass murder. It comes as a bit of a shock to realise when researching Nitram, a film about Australia's most notorious mass killer, that some folks organise their entertainment in this way.

And there's no shortage of films on the subject - films like Elephant by Gus Van Sant. It was based on the murders at Columbine in the US in 1996 that were so horrific until Sandy Hook happened. It is very well made in observational style and it was brave of Van Sant to attempt it. Winning the Palme d'Or for best film that year might have seemed to secure its status, but it isn't as simple as that.

This new Australian film is about the young man who in 1999 shot and killed 35 people and wounded 23 others one long weekend at Port Arthur, Tasmania. Nitram has also been lauded at Cannes, with a best actor award for American actor Caleb Landry Jones, who is compelling in the lead role.

It's from Justin Kurzel, controversial director of Snowtown and True History of the Kelly Gang, who has once again made something technically and aesthetically accomplished, that is also deeply disturbing.

The lead actors here, each inhabiting tricky roles, are excellent. Anthony LaPaglia and Judy Davis as Martin Bryant's parents are wonderful. Based on intelligent writing by Shaun Grant and the cinematography by Germain McMicking, Nitram would be a powerful drama, even if we weren't chasing the clues that mark the main character's journey to notoriety, or waiting for the moment when he snaps.

Of course, there is heaps about Bryant online. That he was diagnosed at age 12 as a paranoid schizophrenic, had a severe intellectual handicap and a personality disorder. But the film Nitram could have been more explicit about these issues, probing Bryant's mental illness. Early on, there's an interesting extract of news footage with the real Martin Bryant as a young boy, when he was in hospital after having burnt his hands while letting off firecrackers. Would he stop now? Nope.

As a young adult he was on a Centrelink disability pension and his issues were being managed with medication. At home with the olds or abroad seeking friendship and attention, the eponymous Nitram (Martin spelled backwards) was at a loose end and affectless. Mum and Dad were at cross-purposes over their son, but there was a sister who lived a reasonably normal life, and she is left out of frame.

Life is crushingly ordinary. Mum makes a futile effort to chase away the wasps nesting under the roof, Dad spends too much time in front of the telly watching Tatts Lotto. It seems like any other place in suburbia. Things at home could be worse, but the lad takes up the offer of a spare room in the rambling, unkempt mansion of Helen, a reclusive heiress in her 50s. A fan of light opera and owner of a multitude of pets, Helen is played by the remarkable Essie Davis.

Some important points about Bryant have been left out or elided. Nitram has his Mum over for dinner before the Port Arthur rampage, but there were girlfriends and one of them was in fact with him that night. Rather than the smiling, gloating loon in the video undergoing police questioning in 1996, the perpetrator presents here as remote, dreamy, misunderstood, with a streak of menace.

The scenes in the gun shop where fistfuls of $50 notes are exchanged for semi-automatic weapons are a reminder of how easy it must have been. If the filmmakers had also asked why the various social security and medical systems failed to respond to clear warning signals, that would have been even more interesting. We knew so much of this, anyway, didn't we?

Although there is a disturbing scene where Nitram roughs up his unresponsive father, there is no gun violence on screen, only gunshots heard in the distance or out of frame. The restraint is laudable, but a strong case is not made for the nature of the perpetrator's mental illness, and it leaves a huge question mark over this strange, sad personal quest for notoriety.

"What is wrong with you?" asks Dad, in a rare moment of anger and frustration. It's the question the film never really answers.