WHAT is "evil"? It’s humanity’s founding question. We’ve struggled with the concept of evil since we clambered down from the trees. What is the Book of Genesis, after all, but a clunking attempt to confront our capacity for transgression and cruelty.

Priests, poets and philosophers have worked their lives away for millennia now trying to understand the dynamics of evil. Is evil innate, irredeemable, unforgivable? What do we mean when we use the word “evil” – that the instigator took some perverse glee in causing wanton suffering, derived joy from pain the way others derive pleasure from a sunset?

Psychiatrist Dr Gwen Adshead is the latest thinker to set her mind to the study of evil. She comes at the subject with decades of first-hand experience. This isn’t a woman who’s honed her understanding in university common rooms or literary salons. Adshead studied “evil” on the wards of Broadmoor.

The Herald:

Adshead’s new book – The Devil You Know: Stories of Human Cruelty and Compassion, a memoir comprised of chilling case notes – leaves the reader with a nuanced, subtle understanding of “evil”. For Adshead, evil isn’t innate, we aren’t born to kill. Evil is constructed – built within offenders, through childhoods of neglect, profound suffering in adulthood, untreated mental illness, and the wilful blindness of a society to the outsiders amongst us falling apart at the seams.

Nor is evil predestined. Two children could be taken at birth and given the same set of circumstances – abuse and a life in care – but one will never offend while the other will. Or perhaps there are two very different children – born into comfort but starved of love, emotionally stunted. Again, one may offend, the other not. Environment provides the seedbed – it’s hard to think of a violent criminal without some profoundly adverse life experience – but it’s the uniqueness of the human soul that waters the seed. Circumstances predispose many of us to crime, but offending is either chosen or not chosen by each of us every day.

From that standpoint, it’s only a hop to the conclusion that “evil” – as we discuss it – doesn’t really exist. Suffering exists, frailty, stupidity, mental illness – and sometimes that dreadful cocktail combusts into an act so awful the rest of us have only one word to explain what happened: evil. To Adshead, those we call evil are “survivors of a disaster where they are the disaster”.

I’ve spent 30 years as a writer investigating what we call “evil”. I’ve interviewed terrorists, mass murderers, rapists, paedophiles, stalkers and extremists of all stripe. In my own layman’s way I’ve come to the same conclusions as Adshead. So many of those I’ve talked to fall into the pattern of “care, addiction, homelessness, violence, prison” – but then so many haven’t as well. Many of the worst offenders had lives of comfort. But deep inside all of them, something was horribly wrong – some wound had never healed, which they decided to act upon. The cruelty of the crimes, a revenge for – or a means of trying to silence – the pain inside them.

If I’m asked about the “worst” human I ever met, it’s not a terrorist or murderer, it’s a sex offender I spoke to in Glasgow’s Barlinnie Prison many years ago. He was 30, a professional who knew how to deploy therapy-room buzzwords – “victim empathy” was a constant theme–- to give the impression of rehabilitation. There was a sense of icy nothingness to him – like talking to a machine pretending to be human. I asked him why he’d done what he’d done and he peddled the usual excuses: the “stressors” in his life (another therapy-room buzzword), alcohol, overwork, an uncaring partner. A billion people suffer the same things every day and don’t rape children, I said. He smiled and replied: “It wasn’t rape. She led me on.”

“Led you on?” I asked. “Yes,” he said, “she wanted to have sex with me. I was weak. Couldn’t resist.”

I looked at the warder in the cell and asked to end the interview. I knew about the offender’s crime, you see. His victim was six. I’m not a person of violence, but I told the guard once we were outside that I’d had to get out of there as I wanted to kill that prisoner. “I understand,” he said. “I feel like that every day.”

If I’d spent more time trying to understand that prisoner, though, I might have tempered my rage. Like the patients Adshead treats, he was a perfect example of the banality of evil. Whether it’s the doctor who downloads images of violent child abuse, the serial killer who trawls London’s gay clubs for victims, the arsonist enraged at her cold mother, the teenager in a girl gang who beats a homeless man to death or the stalker terrorising her victim – there’s a mundanity to the perpetrators Adshead meets. None are Count Dracula – there’s no dark glamour. They’re just ruined versions of you and me.

Adshead has a literary soul. In a book co-authored with writer and dramatist Eileen Horne, she uses the works of poets and playwrights often to help her make sense of the cruelty she confronts. When one man recounts killing his father, Adshead recalls Medea – Euripides’s tragedy of familial murder. The suffering of another perpetrator reckoning with their crimes evokes Lear howling in existential despair.

This affinity for language explains Adshead’s gift as a psychiatrist. She listens to the words criminals use to explain their crimes, in order to “unlock” what went wrong with them. Once that lock opens, the criminal may never be truly “well”, but with understanding comes the likelihood that offending can be drastically reduced. That’s all we can really hope when dealing with criminals of such devastating violence.

Adshead brings a unique insight to female offending – too little studied because of what she sees as the profoundly sexist notion that women can’t be as brutal as men, and that imprisoned women are more victims than perpetrators.

There’s also a quiet social rage here. Adshead is excoriating in her condemnation of a society which lays great emphasis on physical health – on strokes and cancer – but continually slashes spending on mental wellbeing. If we reassessed our priorities, she makes clear, and treated people early – in childhood particularly – then we’d tackle and prevent so much of what we call “evil” in this world we’ve built around us.

The Devil You Know by Dr Gwen Adshead and Eileen Horne is published by Faber, £16.99