The idea of “the banality of evil” was proposed in the wake of the atrocities committed during the Second World War. It was coined by political theorist Hannah Arendt who, while reporting on the trial of the Nazi officer Adolf Eichmann, concluded that evil can be totally arbitrary. Later, the Nigerian writer Chinua Achebe described in his poem "Vultures" how “the Commandant at Belsen” with “the fumes of human roast clinging rebelliously to his hairy nostrils” will still have the capacity to buy his child a chocolate on the way home.

The theory prevails in the war-crime courtrooms of today. For his new book, The Master of Confessions: The Making of a Khmer Rouge Torturer, French journalist Thierry Cruvellier sat through the trial of Kaing Guek Eav, or Duch as he was known during the Khmer Rouge regime, who oversaw S-21 prison at the former Tuol Sleng high school. Duch is so far the only Khmer Rouge leader to have been sentenced for crimes against humanity. An estimated 17,000 people were imprisoned at S-21; some believe the figure to be more like 20,000. Only seven are known to have survived.

Perhaps because he is the only Khmer Rouge figure to have been brought to justice, Duch has long been the subject of fascination and has prompted much discussion. Nic Dunlop’s The Lost Executioner tells the story of how the Irish photojournalist found him in 1999, a converted Christian, working for a charity in Samlot. In Rithy Panh’s memoir The Elimination, the filmmaker interviews Duch about his role in the regime. A reporter for this newspaper last month visited him in Kandal provincial prison, and found him “in a room surrounded by books”, with a fan, sofa and a “good bathroom”.

But Cruvellier’s new book is unique in that it hinges on the brutal and gut-wrenching testimonies of Duch’s victims and their families. Here, as Cruvellier documents the trial, he reflects on the power structures within the regime as well as how it impacted individual families. In the middle of the book, he also considers the role the genocide plays in Cambodia’s tourist industry.

We hear from survivors Vann Nath, Buo Meng and Chum Mey; the families of those he killed such as the New Zealander Kerry Hamill and the wretched professor Phung Ton, who decided to enter the Democratic Republic of Kampuchea from France in order to find his family, and was kidnapped. We also see some of the statements from Khmer Rouge survivors with fading memories who believe they were imprisoned in S-21 but, it turns out, weren’t.

The process of a massive trial such as this, we are constantly reminded, is never a simple one.

Other testimonies come from those who worked at the prison. In Rithy Panh’s documentary S-21: The Killing Machine, prison guard Him Huoy claims that everything he did, he did under orders from Angkar, or the organisation.

He reiterates this in The Master of Confessions. This is a common line among those involved, including the interrogators Prak Khan and Mam Nai, and Duch himself, who points the finger at the head of security, Son Sen: “Back then, you couldn’t challenge it. There was no way out. I had to follow orders.”

Throughout the book, Cruvellier touches on this problem of culpability. No matter how much Duch tries to justify his actions in the name of Angkar, the prosecution tells the court as the trial comes to a close: “You must recognise that he was not a victim of the system but rather its loyal and dedicated agent.” In rebuttal, Duch’s defence lawyer François Roux invokes the Milgram experiment of the 1960s, in which an American psychologist, in the wake of Eichmann’s trial, tried to discover the extent to which people follow orders by instructing them to carry out electric shocks on others. A total of 60 per cent obeyed the person in the white coat. Roux told the court: “I have heard people here in this room tell the court, ‘I can’t answer that, I have to go and check with my superior.’ Yes indeed, that is how we all function.”

Cruvellier is no stranger to war crime tribunals. His book Court of Remorse: Inside the International Criminal Tribunal for Rwanda examines the attempt to bring to justice the perpetrators of the Rwandan genocide. He has also covered tribunals in Sierra Leone and at The Hague. But at the beginning of his new book, he says that in comparison to other war criminals, Duch stands out in particular for having admitted to the bulk of his crimes and co-operated with the court to explain himself and his actions.

Being familiar with analysing the atrocities committed by war criminals, Cruvellier is well equipped to examine psychologists’ studies of Duch’s perplexing character. He uses avoidance, they have said, or compartmentalisation, whereby, just like the commandant in Achebe’s poem, he has the ability to keep different aspects of his existence completely separate. In Cruvellier’s words: “Compartmentalisation is what allowed Duch to be a good father while knowing that children were being murdered at the prison.”

The discussion as to whether the Khmer Rouge Tribunal has come too late also arises. Duch was 69 years old when he was sentenced to life imprisonment in February 2012, decades after he ordered any of atrocities of S-21. At the beginning of Cruvellier’s book, a judge asks one of his subordinates if he is still frightened of him, and he replies, “No.” One is reminded of the recent images of the old and frail Nuon Chea and Khieu Samphan, the court’s most recent defendants. At one point, when a camera zooms in on the feet of a survivor talking about having his toenails ripped out as a form of torture, Cruvellier describes the courtroom as “a circus”.

And yet there is no choice but for justice to be carried out, as much as it can be, Cruvellier argues. How else would we be reminded, crucially, that “becoming a mass murderer is often the uncertain and contingent fate of ordinary men”? The Master of Confessions is chilling, thought-provoking and brilliantly written. Above all, it is a moving tribute to those who suffered at the hands of Duch and his subordinates, and a critical reminder of the cruelties of which humans are capable.

Look out for The Master of Confessions at Monument Books before the end of the month.