In certain parts of Northern Ireland in the late 1970s, a stranger arriving at the door could provoke panic, even terror. The town of Lisburn, near Belfast, was not such a place. Predominantly Protestant and home to many members of the Royal Ulster Constabulary (RUC), it had for the most part escaped the violence that had ravaged other parts of the province. Up until 1977, as Ian Cobain puts it, not a single member of the security forces had lost their life in Lisburn.

All that would change on the morning of Saturday, 22 April 1978, when Millar McAllister, a police photographer, opened the back door of his home in Woodland Park, having glimpsed a figure moving in his back garden. He was shot three times at close range, twice in the chest and the third time, as he was lying on the ground, in the head. In the silence that followed, the killer noticed McAllisters seven-year-old son, Alan, standing just inside the kitchen door, frozen to the spot. They stared at each other for a long moment until the boy started screaming. The stranger ran to a waiting car, the boys cries echoing in his head,

In Lost Lives, the vast book of historical record that chronologically documents every death in the Troubles, Millar McAllister is listed as victim number 2,017. The bare facts of his life are outlined thus: RUC, Protestant, 36, married, two children. In Anatomy of a Killing, Ian Cobain rescues him from the abyss of history, tracing the arc of his short life and contrasting it with the still ongoing, altogether more tangled, life of Harry Murray, his killer.

By reconstructing a single murder its planning, its ruthless execution and its protracted aftermath through in-depth interviews and the careful sifting of not always reliable evidence from official records, Cobain also casts new light on the culture of terrorist violence and state repression that defined Northern Ireland during 30 years of conflict.

Cobain is a seasoned, award-winning investigative journalist (most recently for the Guardian), who also sketches the social and historical context that spawned the Troubles. Throughout, his style is brisk and his tone level-headed, the violence he chronicles often evoked through spartan, but chillingly descriptive, detail. Of the aftermath of the IRA bombing of the La Mon hotel restaurant, which happened on 17 February 1978, just a few months before the murder of Millar McAllister, he writes: Twelve people, including three married couples, died in the blast. All were Protestant. The dead were so badly burned and shrivelled by the flames that firemen thought initially that some of them were children. Hell is in the details.

Amid such carnage, the death of an individual could pass all too swiftly into the anonymous realm of statistics, forgotten by all but family members and loved ones. Cobains book is, among other things, an act of reclamation. It is also, in its skilful telling, a tale of two ordinary lives converging with the inexorability of a Greek tragedy.

Millar McAllister joined the RUC in 1961, when the Troubles, as Cobain puts it, were barely visible on the horizon. He had two hobbies: photography and racing pigeons. The former provided him with a well-paid job; the second unwittingly led to his death. McAllister wrote a monthly column for Pigeon Racing News and Gazette under the byline The Copper, which was accompanied by his photograph. When an IRA suspect, who was being held at the Castlereagh interrogation centre in east Belfast, recognised McAllister from the photo, the word went out to find him. Soon afterwards, Harry Murray was dispatched with another young volunteer to carry out his execution.

In almost every way, Murray comes across as the polar opposite of the level-headed McAllister: impetuous, impressionable and instinctively rebellious. What they had in common is that they were both Protestants, Murray being one of the very few from his community to join the IRA. A few years before, he had been driven out of his home in loyalist Tigers Bay in Belfast by local paramilitaries. His transgression was to marry a Catholic. Having been resettled where his wife grew up in nationalist north Belfast, he grew increasingly sympathetic to the republican cause. Murray seems to have drifted into the ranks of the Provisionals much like, years before, he had enlisted on impulse with the Royal Air Force and served overseas. His military career ended abruptly after one too many breaches of discipline. I just couldnt take orders, he tells Cobain without irony.

Murrays renegade life was not without principle, however. During his induction into the IRA, he claims to have told his recruiters there were two things he would not do: kneecappings and shooting Protestants just because they were Protestant. Like all IRA combatants, though, he regarded the RUC as the enemy in a just war, and, as Cobain discovers, remains remarkably free of remorse for the brutal taking of Millar McAllisters life. In 1983, while serving time for the killing, Murray would take part in an audacious IRA jailbreak from Long Kesh prison, shooting a prison officer in the leg before being wounded himself. On his recapture, he was set upon by prison officers who berated him as a turncoat bastard.

As with Patrick Radden Keefes recent book, Say Nothing, which uses the IRAs disappearance of Jean McConville in 1972 as the starting point for an illuminating exploration of the conflict, Anatomy of a Killing deftly merges history, social context and anecdotal testimony. Cobain explores the psychology of political violence, citing a study from 1978 which found that, rather than being the psychopaths of tabloid headlines, the IRAs political killers tended to be normal in intelligence and mental stability. He also suggests that vengeance may have been a crucial motivating factor for young men joining the Provisionals and, in Murrays case, it is clear that he has never forgiven his own community for the humiliation of his expulsion.

The immediate aftermath of the killing also makes for deeply unsettling reading. On information obtained from an IRA informer, Murray and his accomplices were arrested and taken to Castlereagh, where they were beaten and interrogated relentlessly by Special Branch men working in shifts. Anne, an IRA courier, confesses to her role and, Cobain writes, appears to have suffered a fairly complete physical and psychological breakdown.

The man she gave the gun to after the killing, Brian Maguire, whom Cobain describes as highly strung, was not an IRA member. He was interrogated non-stop for 12 hours and, the next morning, was found hanged in his cell. His death remains disputed. Among the revelations in Cobains book is testimony given at the time by another suspect called Phelim, which provides what Cobain calls an accurate description of the torture technique that became known as waterboarding when used by the CIA in the years after 9/11.

If there is much that is compelling in Anatomy of a Killing, what lingers longest is the awful mundanity of the events leading up to and after the killing. Cobain describes how, on that fateful morning, Anne calmly carried the gun from Belfast to Lisburn on a bus, and, having arrived early, went shopping for a birthday present for her brother. Just a few hours after he killed McAllister, Murray returned to Lisburn to play football on a pitch close to his victims home.

As the Troubles begin to fade into history and forgetting, it is in these incidental actions that the deep moral fracture caused by the conflict comes sharply and chillingly into focus. We would do well to remember how quickly violence can become almost normalised in a culture riven by intractable differences of identity and belonging.

Anatomy of a Killing: Life and Death on a Divided Island by Ian Cobain is published by Granta (18.99). To order a copy go to guardianbookshop.com. Delivery charges may apply

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Anatomy of a Killing by Ian Cobain review a death that casts new light on the Troubles - The Guardian