Snowtown, Justin Kurzel, 115 mins (18)

Charismatic Australian multiple murderer John Bunting was the ultimate stepfather from hell

Daniel Henshall is John Bunting (foreground), one of small-town Australia's most notorious criminals (Revolver Films )

In Australia, we're told, the name "Snowtown" instantly strikes a grim chord.

A series of brutal killings took place in the 1990s, often referred to as "the Snowtown Murders". In fact Snowtown, north of Adelaide, was not where the killings happened – it was just where the bodies were stored in barrels.

But Justin Kurzel's film, graphically violent as it sometimes is, is not out to make us squirm. Nor does it simply provide a forensic account of the crimes of John Bunting and his accomplices. Snowtown is, among other things, about warped family relations, about community, and about the terrible power of the charismatic, approachable leader – the Good Bloke as bloody tyrant.

Scripted by Shaun Grant, Snowtown concentrates on the experience of teenager Jamie Vlassakis, Bunting's accomplice and de facto stepson. We first see Jamie (Lucas Pittaway) as a hulking boy, his barely expressive face suggesting an empty field flattened by boredom and oppression. Jamie and his two younger brothers, we learn, are being abused by neighbour Jeffrey, sometime boyfriend of their mother Elizabeth (Louise Harris). We also see Jamie sexually assaulted by elder stepbrother Troy (Anthony Groves), the matter-of-fact execution of this scene suggesting that this is just part of his daily life.

Then the answer to Jamie's woes seems to turn up in the form of affable hellraiser John (Daniel Henshall), who mounts a whole-hearted revenge campaign against Jeffrey. He becomes a cheerful friend and protector to the boys, who've clearly never had anyone buy them ice cream before – even if it is for the purposes of daubing "Fag" on Jeffrey's house.

Moving in, John soon has friends and neighbours gathered round the kitchen table for discussions of what's to be done about the homosexuals and paedophiles he despises. He's a master manipulator of prejudices and resentments, and these extraordinary scenes offer a sobering demonstration of the workings of demagoguery on the domestic level. Like a rabble-rousing TV chat-show host, John gives everyone a turn at expressing their violent fantasies, then picks out dissenters, implicitly identifying potential victims in the fold. Henshall's portrayal of John is quietly terrifying – beaming an encouraging drinking-buddy smile one moment, the next suggesting homicidal rage behind a placid mask.

It soon emerges that John likes to put his threats into action. And eventually, Jamie finds himself participating in the violence, in the scene that's hardest to watch – a bloody execution in the family bathroom. The killing is directed with painstaking precision by John, and finished off by Jamie; the ecstatic release on the boy's face is one of the most shocking things here.

The film's milieu – the deprived margins of working-class Australia – is photographed with intense quasi-documentary roughness by Adam Arkapaw. But the squalor in this world shows its scars not so much in the trash-strewn streets and mouldering sofas as in the variously dead and angry eyes of the people, who – apart from experienced actor Henshall – are all for real. Kurzel shot his film in the area where the events happened, and cast it with local non-professionals. Everyone looks as if they've seen their share of frosty mornings: a haggard transvestite resembling Freddie Starr in a nightie and tattoos; John's silent lieutenant, a hatchet-faced, tracksuited embodiment of death; and Elizabeth, delicately played by Louise Harris who, for whatever reason, has seen the lower half of a handsome face crumble into battle-weary dereliction.

As for Lucas Pittaway's Jamie, disarmingly vulnerable with his numb, doughy features, it's one of those cases in which you're not quite sure whether you're watching a great performance or simply a case of a director wheeling in someone with the right kind of physical presence. But Snowtown never feels as if it's exploiting its cast; it avoids the trap of staring-at-the-hicks condescension. Still, Kurzel can't quite resist the temptation of gratuitous weirdness – as when John watches an obese neighbour undress among her rows of dolls and toy dogs.

An unsettlingly fragmented structure is riddled with gaps and unanswered questions – eg, where did John spring from in the first place, and how much did Elizabeth know? At only one point does Snowtown lay its cards on the table in terms of wider issues. During one of his hate-stoking sessions, John calls brutality "an Australian fucking tradition", referring to the commemoration of the armed forces on Anzac Day, when "the whole country applauds a bunch of men who kill and torture blokes". Snowtown is so precise in its evocation of a specific world in which slaughter flourished, that the film would lose something if you were to see it as a flat "state-of-Australia" statement. But overall, Snowtown is quite something – a descent into hell evoked with chilling sobriety.

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