Notably absent from Sotiris Dounoukos' film, Joe Cinque's Consolation, is the voice of Helen Garner. In her best-selling book about the death of 26-year-old Joe Cinque and the murder trial that followed, we see everything through Garner's eyes. She makes us party to her own fears and recriminations; her battles to be fair to Joe's girlfriend, Anu Singh, who injected him with a lethal dose of heroin. We share the author's sympathies for the victim's family, and her bewilderment at the workings of the law.

It is this insistent, questioning voice that makes the book so compelling. We feel as if we are discovering the facts at the same time as the writer, with the same mixture of shock and revulsion. The tale unfolds like a bad dream, with every character apparently incapable of the most basic sense of empathy.

In bringing the story to the screen, Dounoukos has removed the authorial tone and the accounts of the trials of Anu Singh and her friend, Madhavi Rao. There is little room for the parents of Joe and Anu, and their contrasting forms of pain. He has replaced Garner's highly subjective approach with the would-be objectivity of a documentarist.

We begin in October 1997 with Anu's garbled 000 call that summoned an ambulance when Joe was already too far gone. Soon we are in flashback mode, tracing the history of the fatal relationship as if it were a mildly diverting soap opera. It's a story that gets weirder and more disturbing by degrees, until you want to yell: "What's the matter with these people!? Why are they so passive at the thought of suicide and murder?"

Murderous delusion

The film takes pains to bring Joe Cinque alive in a way never attempted in the book. Jerome Meyer plays Joe as an easy-going, likeable guy, devoted to his difficult girlfriend. We see Joe at work, and eavesdrop on conversations he has with friends.

Maggie Naouri's portrayal of Anu is the more difficult task, as her personality defies all stereotypes. Deeply narcissistic, probably depressive – borderline psychotic? – Anu had an incredible ability to bend others to her will and draw them into her obsessions. Her belief that she was suffering from an incurable wasting illness was unshakeable, even though numerous medical tests had turned up nothing. When she decides to commit suicide, it is as if she is preparing for a role in the school play. It is a piece of theatre in which she has reserved for herself the starring role.

This may be a partial explanation for the inertia that grips everyone in her circle. The idea of a suicide or a murder-suicide, seems too far-fetched to believe, while Anu herself is an obvious fantasist.

When she announces she's going to take Joe with her it's hard to know whether she is motivated by revenge or merely spite. She claims he bought her a weight-loss drug – at her own request – that is responsible for her mysterious illness, but there's also a suggestion that it would be a mercy to kill him as he'd be so traumatised by her loss. Either way, it's a unilateral decision.

We watch with growing amazement when she visits a fellow law student to buy a lethal dose of heroin as if she were borrowing a textbook. The supplier gives her instructions on dosages, reasoning that her plans for suicide and murder are none of his business. Strangest of all is her friend Madhavi (Sacha Joseph), who assists in all the preparations for the crime, with no apparent scruples.

Stew of moral complexities

The events are so well known, and indeed established in the first scene of the movie, that I'm not going to worry about spoilers. It's history that the deed was stretched out over a week, and two separate dinner parties. Joe survives a first doping, largely because the heroin became congealed in the needle. On the second attempt, it takes Anu all weekend, and an extra dose of heroin, to kill him.

It's also history that Anu was given a 10-year sentence for manslaughter, not murder, which with time already served and parole, came down to two-and-a-half years. During that stay she was allowed to attend day classes at Sydney University, and complete a master's degree in criminology. Her accomplice, Madhavi, was acquitted of all charges.

These facts, which for many people seem to prove the law is an ass, are not included in the narrative, only as a final statement flashed on screen.

It leaves us with a sickly feeling that there is something very wrong with a community – albeit a student community – in which nobody feels any responsibility to protect an innocent man from his girlfriend's murderous intentions. The elaborate planning and dogged execution of the scheme make it hard to accept the idea that Anu was temporarily out of her mind. It reveals a judicial system more concerned with the welfare of the killer than the wrong done to the victim and his family.

The film leaves us in a stew of moral complexities overshadowed by the insistent fact of Joe Cinque's death. When the person responsible for the crime is treated so compassionately we see the terrible price that everyone else has to pay.