I read and watch more true crime than crime fiction, yet that enthusiasm is something I wrestle with. It isn’t lost on me that real life atrocities pass quickly into the realms of entertainment, and I can only imagine how appalling that must be for anyone enduring what is already a devastating experience. When I started work on my new book The Mysterious Case of the Alperton Angels my first thought was that this novel could work as a dark satire on that very dilemma.

American author and journalist Michelle McNamara was very much front of mind. Having written the true crime classic I’ll Be Gone in the Dark, McNamara was commissioned to write a book about several unsolved home-invasion rapes and murders across California in the 1970s and 1980s. She discovered that crimes previously attributed to several different serial murderers were the work of just one man – a figure she dubbed The Golden State Killer.

The book and HBO documentary of the same name (directed by Liz Garbus) are tributes to McNamara’s passion and commitment, but along the way her dedication morphed into something that would ultimately prove self-destructive. Determined to the point of obsession – both to finish the book and hunt down the killer – her twin aims proved mutually exclusive. She accumulated an overwhelming amount of information that trapped her in an impossible loop – she couldn’t solve the case, so couldn’t finish the book.

McNamara was found dead on April 21, 2016, her death ruled an accidental overdose occasioned by dependence on opioids. The book was published posthumously in February 2018.

I can understand something of how McNamara must have felt, both about her book and the case – and how the conflict between those tasks would have had a depressive and wearing effect. Curiosity and fascination can so easily become obsession, and I feel a great deal of empathy with those who have paid the ultimate price in their pursuit of a story. Because when you consider McNamara to be the Golden State Killer’s final victim, her story is by no means an isolated tragedy. Veronica Guerin, Marie Colvin, Anna Politkovskaya, Lyra McKee, Daniel Pearl among many others are all names that represent something I wanted to capture. For The Mysterious Case of the Alperton Angels I knew my main character would be a true crime author with a similar level of commitment to their work. But there was a problem …

My first novel, The Appeal, is structured as emails and texts that fly between characters as two volunteer medics return to England and join a community theatre group. In my second, The Twyford Code, a former prisoner is determined to investigate a traumatic event in his youth and documents it via his iPhone’s voice recording app. Both are considered “cosy” crime. Would readers want my third novel to have such a serious underlying message?

Cosy crime might mean different things to different people, but for me it’s not about shying away from the dark side – it’s more about focusing on the mystery and solving the puzzle. I liken it to the way a police detective, appalled as they are by the crime they are investigating, must put aside their emotions to get to the bottom of the case.

My main character needed a case to become obsessed with. For this, I was drawn back to a script I’d written many years before. About a small cult of men who believed themselves to be angels sent to earth to destroy the new-born anti-Christ, its story had never left me, but when I re-read the script, it felt like something that had happened a long time ago. That was when two became one: my journalist can investigate an historical case… why? Well, because the baby who escaped the clutches of the cult back then would finally be an adult and able to tell their story. Only, the subject of cults is a harrowing, sinister place indeed and I didn’t anticipate the quality of darkness I would discover there…

At about this time I watched the HBO documentary The Vow and became fascinated by NXIVM (pronounced Nexium) a cult that masqueraded for years as an intense personal development and multi-level marketing programme, based in Albany, New York. The lure of this organisation for so many highly educated, professional and intelligent young people is difficult to quantify in the cold shadow of hindsight. It was certainly not a religious or ideological belief system. Instead, its ethos seemed to centre entirely on the self-improvement teachings of its “guru” Keith Raniere.

Former cult members spoke of his magnetic charisma as he courted wealthy young people and those on the fringes of Hollywood. Entire families were drawn in and all the time Raniere was manipulating and abusing women in plain sight. Something powerful had kept so many in thrall to this increasingly toxic organisation – one that finally resorted to branding its members. But his charm isn’t something that comes across on screen. How do you capture that intangible x-factor that makes a single individual so all-powerful and influential?

Raniere is currently serving 120 years for, among other offences, sex trafficking, racketeering and forced labour. Several other NXIVM leaders are also imprisoned, yet it’s clear most, if not all, had joined with genuinely benign and well-meaning aims. Something about the way power corrupts sees people descend into cruelty and manipulation as they climb the ladder of approval and privilege that defines a cult. Once ensnared, factors such as peer pressure, pride and other subtle means of social control make escape feel impossible. This is how predatory individuals exert coercive control on a grand scale and it seems that degree of abusive behaviour can reproduce itself in some of those who succumb to it.

We share a common delusion that it’s other people who join cults, other people who fall for “crazy” gurus and extreme ideologies. My research revealed that isn’t the case at all. It’s not certain people who are vulnerable – we’re all vulnerable in certain situations.

This story is about the psychology of faith and belief, the human need to belong in groups with a common aim, and the strange co-dependent relationship leaders of all kinds must foster with their followers.

The Mysterious Case of The Alperton Angels by Janice Hallett is published on January 19 by Profile Books, €16.99.