Piers meets a psychopath who killed his sister to hurt his mother. What kind of person, I pondered, could be deemed such a threat to my life that I wouldn’t even be permitted to occupy the same airspace?

You’re not allowed to sit with him face to face,‘ briefed Stuart Cabb, who runs the TV production company that makes my crime documentaries. ‘Why not?’ I asked, mindful of the fact that I have previously sat face to face with numerous horrific serial killers and murderesses.

He’s too dangerous,‘ came the reply.

Wow. Given the extremity of violence committed by some of my interviewees, this was an extraordinary statement.

‘So how will I speak to him?’

‘He’ll be in a secure room, behind heavily reinforced glass and monitored by armed guards.’

‘No direct contact at all?’

‘None. The prison has been unwavering about this. It’s for your safety.’

Paris Bennett, from Abilene, a town in rural Texas. Bennett has an IQ of 141, which qualifies him as a genius. Less than a quarter of one percent of the world’s population has that level of intelligence.

He is also a psychopath. I don’t mean someone with psychopathic tendencies, I mean someone who has been formally diagnosed as a psychopath by medical experts.

True psychopaths have a chronic mental disorder that manifests itself in a number of personality traits including amoral or antisocial behavior, extreme egocentricity, a lack of ability to love or establish meaningful relationships, and no sense of guilt, shame or embarrassment.

They are invariably callous, with a complete disregard for the feelings of others, and often use charm and habitual lying to manipulate or coerce others into doing their bidding.

Psychopaths can also be quite terrifyingly violent. Paris Bennett ticks every box.

Twelve years ago, when he was 13, Paris decided he wanted to punish his mother Charity for various irritations he perceived her to have caused him. His first thought was to simply kill her.

Then he devised a new, shockingly depraved plan: he would murder his four-year-old sister Ella instead, knowing he would then go to prison so his mother would lose both her children at once, and have to live with the horror. It was a plan he would execute with quite sickening violence.

While Charity was at work in a local bar, Paris persuaded his babysitter to go home. He then calmly walked into his sister’s bedroom where she lay sleeping and began ferociously attacking her. He beat and choked her, before stabbing her with a kitchen knife 17 times.

Paris then called a friend on the phone and chatted perfectly normally to him for six minutes before calling the police, who came and arrested him. The murder achieved its purpose.

As Charity told me when we met: ‘If Paris had killed me as he originally intended, I’d have only suffered for a brief few moments.

‘But by killing Ella instead, he knew he was sentencing me to a lifetime of suffering.’

Paris is serving a 40-year jail term, the maximum sentence available for a juvenile in Texas.

‘I felt so vunerable’ Sheila Mwanyigha talks death threats

Now 25, he is up for parole in just a few years’ time. And that is where this horrifying story takes an even more sinister and disturbing twist.

Because, astonishingly, Charity has forgiven her son, says she still loves him and visits him regularly. But she fears that if he is granted parole, he will come and try to torment her all over again.