Kevin Khatchadourian is a mass murderer. And We Need to Talk about Kevin is ostensibly going to tell us about him. But unlike many fictional or semi-fictional accounts of spree killings, e.g. Badlands (1973), Natural Born Killers (1994), or Elephant (2003), Kevin gives us practically nothing from the killer's viewpoint. Unusually for a murder narrative, it neither falls back on 'some people are just born evil' nor attempts to provide psychologically convincing explanations. Instead, this lack is the mystery, the hook that drives the story forwards. But can we glean all we need to know about Kevin from this sad, elliptical narrative of his mother's despair? Can the film provide satisfactory insight without explanation?

Kevin is the long-awaited third feature from Lynne Ramsey; it's been nearly nine years since she adapted Alan Warner's Morvern Callar. Lionel Shriver's novel is also a psychological study of a troubled woman, but stylistically and structurally very different. The eponymous Morvern Callar drifts through life guarding her thoughts, her motivations obscure, hidden from the reader by Warner's dreamy prose. In contrast, Kevin's mother Eva obsessively chronicles her life in intensely self-critical letters to her husband Franklin, searching for the detail that might explain Kevin to him and to herself. This adaptation is more complicated, riskier. Film and book ask altogether different questions.

Shriver's novel is in part an enquiry into how a sociopath might come into being in modern America, and in part a meditation on motherhood. Eva's son became a killer at 15. He's claimed a place in history, choosing to carry out a school massacre in the knowledge that society finds this form of mass murder especially interesting and transgressive, and thus has a disproportionate need to explore it in art and attempt to make sense out of it. Which the book duly does. But Ramsay's film denies this need. It isn't really about sociopathy or parenthood. It shifts the focus of the enquiry to the aftermath - essentially it asks, how much grief and loss can one person bear? 'What made Kevin a killer?' is recognised for the unanswerable question it always was, and left alone; unlike Eva, the film doesn't have to keep wondering why.

The most obvious counterpoint to Kevin is Gus van Sant's Elephant, a fictional account of a school shooting based on the Columbine massacre of 1999. It's a very different approach: Elephant drops us in medias res, following students Alex and Eric in the days immediately before their killing spree and lingering over the murders themselves. Kevin somewhat recalls its languid dreaminess, but Elephant's scope couldn't be more different - it focuses on the massacre to the exclusion of almost everything else. Scraps of context - glimpses of bullying and the killers' families - are the only explanation we get. By contrast, Kevin shows us scenes spanning Kevin's whole life, before and after, and the massacre itself is told almost entirely in aftermath. Yet we're probably offered more of an explanation of Alex and Eric's actions than we are of Kevin's.

Wisely, Kevin's screenplay all but abandons the epistolary structure of the novel; a lesser adaptation might have used voiceover, but all that remains of the letters here are ghosts - flashbacks. In the book, Eva makes much of the civil prosecution brought against her by the mother of one of the murdered children; the court enquiry into her alleged maternal negligence is the formal aspect of her own, much harsher, investigation of herself. Ramsay's film bypasses all this completely; whether or not we want to judge Eva's fitness as a mother is of no interest to it at all. It knows that such judgement doesn't hold the answer we're looking for.

Of Kevin, too, the film asks different questions. As to why he is the way he is, all we get is the opportunity to infer that Eva wonders what she could have done differently. We witness memories she might regret - but these scenes are open to interpretation. The title may express her need to talk about Kevin, but she's never actually able to do so. Kevin seems, to Eva, to create a complicity between the two of them. From an early age, he controls and manipulates her, and her shame at letting him do so keeps her silent. It's an elegant, economical shortcut to get us to the same place as Eva's long investigation in the book: the understanding that we can't know what caused Kevin's nature and that Eva is at heart ambivalent about accepting blame, no matter how critically she may observe her own actions.

Instead of describing the causes of the crime, the film tells the story of Eva's responses to it, through often dreamlike imagery. Most striking are the visual metaphors for bloodshed that recur throughout, standing in for the massacre we barely see and privileging emotional reaction over intellectual interpretation. A younger Eva attends La Tomatina festival in Spain, one of a sea of bodies floating in bright red juice. Crimson enamel is splashed across her house and car by malicious neighbours, and her efforts to remove it leave her covered in streaks of red. Red light frequently saturates the screen. In contrast, flashbacks to before the murders are mostly yellow - the mustard walls of her office, the empty beige spaces of the family home. Where they collide, and events seen in flashback bleed their emotion through into the present, yellow and red mingle. Orange and red lights spangle a rain-soaked street. Tomato sauce splashes across yellow omelettes from eggs broken by a grieving mother in the supermarket. (Food, so often the visible conduit of maternal love, is disgusting throughout.)

Kevin himself is little seen, and actually his portrayal is the weakest aspect of the film. In many of the scenes featuring the adult Kevin (Ezra Miller), we barely see his face; when we do, he's brooding or leering in a way that sometimes slips into the theatrical. Not that his presence is ineffectual; he conveys a powerful, lazy-eyed, sexual obsession with his mother that has a nicely menacing frisson. The children who play Kevin as malevolent toddler and spiteful boy aren't bad either, but again, the simmering hatred they emanate feels over the top compared to the excellent, nuanced performances of the adults (Tilda Swinton and John C Reilly). The closest insights we get into how it feels to be Kevin relate to his physical body; he craves discomfort. Little scratchy details get under your skin, giving a tiny sense of the weariness of being him. In an early prison visit, Kevin bites his nails and arranges the hard crescents in front of Eva. These small symbols of his feelings reflect the shard of eggshell in Eva's mouth, the remnant of spite bred by bereavement and blame.

Essentially, Kevin before is just less effective, less real, than Kevin after. As the embittered juvie prisoner, his fifteen minutes of fame long ago spent, he's left enduring regular visits from the one person whose opinion matters to him - and whom, for that reason, he hates most of all. At this point he finally has an explanation. Having committed murder defines him in a way that being on the verge of it, and the act itself, never could.

In choosing to examine how grief and loss devastate a life, rather than how violence is born, Ramsay's film achieves a balance between literary and visual storytelling. Guilt and blame, before and after, simple love and complex hatred, crime and punishment, beauty and ugliness, life and death, each of these axes pivots on a fulcrum; none come down on either side. By visually dissecting Eva's wordless grief and eschewing rational understanding of Kevin's actions and character, it asks: how can you ever expect to explain someone like Kevin, except through the unsatisfactory prism of other people's feelings? Kevin himself sits like a lacuna at the centre of the film, a blank void, daring us, and Eva, to try to interpret something that simply isn't there, to explain the inexplicable. That's it. That's the insight. The resolution to the mystery of Kevin is that there is no resolution.