Some movies are a tough watch, but still worth watching. That rule is rather apt for Lynne Ramsey's devastating You Were Never Really Here, a darker-than-midnight descent into the most unthinkable of underworlds. It's a film about the very worst aspects of humanity, and it's visceral, uncomfortable and unrelenting. Yet it contains a central challenge to anyone who considers themselves moral, and perhaps an even greater one for anyone who wants to put the Christian faith into fullest practice.

Joaquin Phoenix plays Joe, a hammer-wielding hitman, abused as a child and now specialising in rescuing abused and abducted children. The people who hire him pay handsomely, not just for his skills and success, but for his brutality in dealing with the culprits. He is sent into places where profound evil has been committed, and he brings liberation through rampaging killing sprees. It's not just hard to watch because the violence is graphic (actually director Ramsey chooses to exercise restraint most of the time), but because as Phoenix slays pedophiles and the men who profit from them, we can't help finding a sense of justice and even delight in their murder.

Phoenix – recently seen as Jesus in Mary Magdalene – is mesmerising in the film's lead, a complex mumbling character who is simultaneously a powerful grown man and a trembling little boy. Ramsey has him drifting through a waking nightmare, unsure if he can keep hold of his sanity, and frequently flirting with suicide as he carries out his deadly missions. Jonny Greenwood's jarringly brilliant soundtrack adds to the sense that we're trapped in the nightmare too; in a sense it's a film that's determined to make us aware of just how evil the world can be.

And so as the story advances, and Joe's mission switches from liberation to revenge, we're challenged to consider that old philosophical question: can two wrongs make a right? As Christians we know the 'right' answer: Jesus tells his followers in Matthew 5 v 39 to 'turn the other cheek', while Paul and Peter both instruct us to 'not repay anyone evil for evil.' Ramsey's film forces us to interrogate this kind of idealistic thinking, and ask if we really believe in it. By employing his shocking methods, Joe rescues innocent children from the most horrific situations. Yet as he does so, he pronounces a death sentence on anyone who stands in his way. Ramsey deliberately chooses the most taboo and emotive kind of crime in order to ask us: do certain ends justify any means? And while we believe two wrongs don't cancel each other out, isn't that better than allowing the greater wrong to continue?

You Were Never Really Here is a gruelling, gory film, containing only the briefest glimpses of beauty and light. But it's an important piece of work; both a fine piece of film-making and a challenging piece of provocation. It forces those of us who believe in the idea of grace – that no-one is beyond redemption and the love of God – to consider just how far we're prepared to extend that idea. Does it extend all the way to the 'vilest offender'; the men (or women) who commit the most unspeakable acts against the most vulnerable? Or would we rather those people received a visit from a hammer-wielding Joaquin Phoenix? Our honest answer to that question will give us an insight into our own theology of grace. A film that does that is way more than a simple revenge thriller.

Martin Saunders is a Contributing Editor for Christian Today and the Deputy CEO of Youthscape. Follow him on Twitter @martinsaunders.