Grotesque. Unbelievable. Bizarre. Unprecedented. The then Irish prime minister Charles Haughey famously used these four words at a press conference in the summer of 1982, when a double murderer, the subject of a high profile nationwide search, was found to be staying as a guest in the seaside penthouse of the attorney general, Patrick Connolly. The most wanted criminal in Ireland was occasionally chauffeured around in the state car provided to the Irish government’s chief legal adviser, complete with a garda driver.

It was Conor Cruise O’Brien who shortened this into the acronym that was to define an era: Gubu. The ensuing scandal cost Connolly his job and contributed to the fall of the Haughey government later that year. Picking up Mark O’Connell’s remarkable new book about these murders, I was half expecting social analysis, and perhaps some theoretical reflections on 1980s Ireland, such as Fintan O’Toole offers when dissecting the case in his recent autobiographical social history, We Don’t Know Ourselves. Yet while O’Connell tells the story of the crimes, their background and their fallout with painstaking care, his approach is too restrained and self-searching for state-of-the-nation diagnostics or political allegory around class and power. He has produced a profound meditation on violence and its roots, on the skeining of barbarism and high culture, and on our urge to make sense of chaos and brutality.

Malcolm Macarthur had plenty of what we would now call privilege. He inherited enough from his family’s small estate in Meath to indulge his social and intellectual fancies without the burden of daily work. A handsome, well-spoken man, he was a suave denizen of Dublin’s bohemian bars. Yet however high born, his crimes were lowly and squalid. He had run into financial trouble as a result of his spendthrift ways and, aghast at the prospect of losing his independent lifestyle, conjured up a bank heist that never even made it to the bank, ending up as a tragic and brutal farce. Over the course of a weekend in July he took the lives of Bridie Dargan, a nurse whom he bludgeoned with a hammer in Phoenix Park when stealing her car and three days later, Donal Dunne, a farmer, whom he shot in the face. Both his victims were 27 years old.

Macarthur is the loose inspiration for Freddie Montgomery, the protagonist in John Banville’s 1989 novel The Book of Evidence, but while Montgomery is an aesthete and art collector, Macarthur is more avowedly scientific – well read but a student of economics and analytic philosophy, not fiction. His cultivated rationalism becomes a leitmotif in this book, expressed both in the story of what he calls his “criminal episode” and the often frustrating opacity of the authorial interviews with him.

O’Connell begins by describing his hunt for Macarthur who, after his release from prison on licence in 2012, had taken to turning up unexpectedly at book launches and seminars in Dublin, like a Baggot Street Banquo. Trinity College is a favoured spot, predictably enough; a place O’Connell himself studied, writing his PhD on the works of Banville. After his release, Macarthur is alleged to have applied unsuccessfully to read English as a mature student. At one point, deliciously, he even turns up at an event where Banville is the guest speaker.

O’Connell walks the streets hoping to run into his now white-haired quarry. The lead that allows him to find Macarthur is an article in the Irish Sun that sounds as if it might have been published in the Onion: the headline read “Masking a Murderer: Double killer Malcolm Macarthur backs Covid lockdown restrictions – labelling them ‘necessary precaution’”. Gubu indeed. But also, as becomes apparent in O’Connell’s later interviews with Macarthur, entirely in keeping with his scientism and prosaic practicality.

When he finally meets Macarthur, stopping him on the street, O’Connell rather sheepishly identifies himself as an “essayist” in an attempt to distinguish himself from the sort of true-crime hack who might put Macarthur off. Yet to my mind, it’s a deadly accurate self-description. O’Connell’s writing, like the essay form, sits on the cusp of faithful reportage and personal reflection, objective retelling and creative obliquity. Here is a meticulous attempt to bear witness, to find the truth of what happened and to explain Macarthur’s behaviour by fleshing out his life and background. Yet O’Connell also relates, with lyrical candour and sensitivity, his own familial connection to the case, his struggle with the ethics of writing about a murderer, and his anguish at the ineffability of his quest – of the “sullen and persistent silence at its centre”.

The story is compellingly told, with the structure and pacing taut, the writing deft and limpid, all marked by an absorbing honesty and ethical concern. Crucially, the moral intelligence with which he treats the themes, including the plight of the victims, explodes the idea that the criminal is interesting for reasons of diabolical glamour. Macarthur compels him not as an individual but rather as a cipher, an “emissary” and “avatar”. “My relationship with him is, after all, an inherently extractive one,” he writes, “as though I were a prospector that has struck a rich vein of crude oil. With this, I can reconcile myself. Because to understand Macarthur, or to attempt to do so, is to understand the darkness and violence that run beneath the surface of so many lives, and which have shaped so much of human experience.”

O’Connell’s literary persona is warm and winning: curious and tenacious, yes, but also modest, hesitant, self-querying. Nonetheless, this book is an outstanding achievement, and a worthy addition to literary attempts to understand the human propensity for evil.

• A Thread of Violence: A Story of Truth, Invention and Murder by Mark O’Connell is published by Granta (£16.99). To support the Guardian and Observer, order your copy at guardianbookshop.com