On the evening of 20 August 1989, brothers Erik and Lyle Menendez walked into the den of their home in Beverly Hills, where their parents were watching a film, The Spy Who Loved Me, and they shot them at close range with a shotgun. They were sentenced to life imprisonment without parole and, for years, their story largely faded from the spotlight.

Then, in September, they returned to the headlines following the release of a Netflix drama series and documentary about what happened. Now their case is under review because of new evidence that was not presented at their trial.

Last Monday, 28 years after their last courtroom appearance, the brothers teleconferenced into a hearing from prison, during which their aunt pleaded for their release. “I think it is time for them to go home,” she said.

Their uncle, meanwhile, has called the brothers “cold-blooded” and believes they belong behind bars for life.

What struck me, as I watched all of this unfold, were the opposing ways that different people, even their own family members, portrayed them. Are the Menendez brothers, to borrow the name of the Netflix drama, really “monsters”? Or is it possible that they have changed, as their aunt claims?

In my 30 years as a forensic psychiatrist and psychotherapist working in psychiatric hospitals and prisons across the UK, including Broadmoor, I have spoken to hundreds of criminals who have committed terrible offences in an attempt to help them take responsibility.

Some people assume that this is an impossible task. I’ve been asked: “But surely they can’t be helped? Aren’t they born that way?” The implication being that only an abnormal monster could inflict dreadful damage on another person – or that killers, from Ted Bundy and Rose West to Harold Shipman and the Menendez brothers, are somehow not human.

Certainly, when I first started working in this field, I assumed that people who have committed violent and murderous acts are very different from the rest of us.

But I no longer think this.

What I’ve learnt is that the real causes of violent minds – a subject I examine in The Reith Lectures, which are broadcast in four episodes on Radio 4 – aren’t depicted in true-crime dramas or courtroom transcripts.

The reality is far more complex than labelling someone as simply ‘evil’, as I discovered firsthand.

In 1996, soon after I’d started at Broadmoor while completing my psychotherapy training, I took on a patient called Tony. He had killed three men and decapitated one of them.

I’d read a lot of lurid reports about serial killers but at the time there was little advice available on how to talk to one or offer them therapy, and part of me wondered if there was any point. How would we know if he was “better”?

He was 10 years into his sentence and had recently been stabbed with a sharpened toothbrush by three other prisoners. A suicide attempt had followed.

In our first session, there was silence. He folded his arms and avoided meeting my eyes. When he looked up, his eyes were so dark they appeared almost black. He was suffering from depression and nightmares. “I was thinking that it’s peaceful in here,” he said eventually, breaking the silence. “There’s a man in the room next to mine who keeps shouting in the night.”

It took him months to open up about his recurring nightmare. In it, he was strangling a young man who morphed into his father. It led us to discuss his offences and his family and how, as a child, Tony had suffered violent abuse at the hands of his father; in turn, he began to bully others.

Later I learnt that the man “in the next room” who shouted at night was Tony himself. I suggested that perhaps he was shouting the things that he could not express. He dropped his face in his hands, muffling his voice. “No… I don’t want to,” he admitted. “I can’t be so weak.”

I worked with Tony for 18 months and came to feel compassion and respect for his honesty, even as I still held in mind the terrible trail of destruction he had caused. The fact he’d requested this therapy himself was also a sign that part of him was ready to be vulnerable.

That early experience taught me that no matter their history, if people – including serial killers – are able to be curious about their minds, there’s a chance that we can make meaning out of disorder.

When it comes to serial killers it is generally assumed that they are psychopaths, but I wasn’t convinced that applied to Tony. Psychopaths are unlikely to request help as they don’t want to do anything they’d consider to be demeaning, so on that basis alone Tony wouldn’t have met the criteria, as he had asked for therapy.

The psychopaths I’ve encountered in my career have been neither exceptionally bright nor socially able, nor at all charming. They are usually so lacking in empathy that they cannot see the effect they have on others.

And contrary to common belief, very few killers are in fact psychopaths, especially domestic homicide perpetrators like the Menendez brothers.

Tony’s story also highlighted the role that childhood adversity can play in violent crime. The Menendez brothers argued that they were victims of physical and sexual abuse at the hands of their father, a defence that was challenged in court before they were handed life sentences.

Yet a significant proportion of the population have experienced severe childhood trauma – as many as 10-12% of people in the UK according to some studies – but a far smaller number commit acts of criminal violence.

Which begs the question, what makes some people respond to childhood trauma with violence, while others do not? Could it be that those people are indeed “monsters”? Or, as some of my patients previously put it: “I have done evil things, but does that make me evil?”

There is no scientific evidence that people are born “evil”. And in my experience, there is no such a thing as an evil person – instead, there are evil states of mind.

So, typically, I begin my answer by telling them that it is possible for anyone to get into this state of mind, which is dominated by ordinary emotions of hatred, envy, greed and anger.

Deep down most of us have a capacity for cruelty but the risk factors that make some people act that out with extreme violence are specific. They are a little like the numbers in a bicycle lock. Just as all the numbers have to line up for the bicycle lock to open, multiple risk factors are usually in place before violence erupts.

The most common risk factors are being young and male (with higher rates of aggression and impulsivity); being intoxicated with drugs and alcohol; having a history of family conflict and breakdown; and a history of criminal rule-breaking. Being in a paranoid state of mind caused by mental illness can also be a risk factor, though this is more rare.

The most important risk factor for murder, however, is the nature of the relationship with the victim, especially a history of relationship conflict. It is well known that women are most commonly killed by male partners or family members, and most children are killed by their parents or step-parents. The killing of strangers is rare, and these tend to be cases where perpetrators are severely mentally unwell.

So the first two numbers that align in the bicycle lock could be sociopolitical, and the next two might be specific to the perpetrator.

The final number that causes the lock to spring open can be something that happens between the victim and the perpetrator – whether an offhand comment, an action perceived as a threat, or something as simple as a bad football result. (Domestic abuse soars by 38% when the England team lose, according to research by Lancaster University.)

When the bicycle lock clicks into place, what is unleashed is often a wave of overwhelming emotion that distorts how the person sees everything.

The good news is that over the last 20 years there has been a fall in homicide rates in the UK and elsewhere, which is largely a result of changes in some of these bicycle lock factors.

“The decline of homicide rates since 2004 in the UK – which has also happened in the US, Spain, Italy, and Germany – is partly due to changes in lifestyles such as reductions in binge drinking and cannabis consumption among adolescents,” says Professor Manuel Eisner, director of the Institute of Criminology at the University of Cambridge.

“[It is also] partly the influence of technologies such as mobile phones and CCTV cameras, which add surveillance and opportunities to find help in situations of danger.”

In addition, he attributes the drop to wider shifts including the bolstering of cultural norms opposing bullying, and violence against women, girls and children.

And while there is a minority of people whose minds can’t be changed – who will always be a risk – by paying attention to distorted narratives in the majority of cases, we can find ways to change those violent minds for good.

In 2004 I met a man called Jack who had killed his mother when he was in his 20s. He had been found to be suffering with paranoid schizophrenia at the time, so he was sent to hospital for treatment.

Later, he joined a therapy group that I was running at Broadmoor Hospital. In the hour-long sessions the group members, who had all killed family members while mentally unwell, would talk about how they could avoid violence in future. Jack didn’t always seem engaged but after a year or so, just after another member had talked about past regrets, he spoke abruptly.

“I wish I could say sorry to my mum for what I did,” he said. “I know I was mentally ill, but I wish I could say how sorry I was and that she could forgive me. I hope she understands how much I regret it.”

By seeing themselves in other offenders, some group members were able to learn how it had been possible to delude themselves into thinking that someone had to die; and how waves of anger, shame and fear could lead them to misinterpret actions and words.

Jack seemed more engaged after that day and his mental health improved enough for him to move to a less secure hospital for further rehabilitation.

Group therapy takes time, but afterwards many other men were also considered safe enough to move to less secure treatment facilities, which is a sign of improvement and something we only do if we determine that their risk of reoffending is negligible. Most importantly, they also learnt to take responsibility.

Jack helped me realise that people who kill are not mindless monsters who are born that way. He was an ordinary man who had done an extraordinary thing, as with many others.

None of this is an excuse for violence – and every violent crime is a tragedy for all who are involved – but monstering people is not helpful. It is simply one way to deal with rage and fear. And we miss a chance to reduce and prevent violence if we write off everyone who has murdered or abused in that way.

It takes a radical kind of empathy to sit with a man who has decapitated his partner, or a woman who has stabbed a friend. But trying to comprehend them and gain new insights about ourselves requires going where they walk, and seeing what they see. And that is what ultimately leads to change.

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