PIERS MORGAN meets a psychopath (through reinforced glass) as clever as he is ...

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.'

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?

The answer was 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 per cent 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 behaviour, 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 baby-sitter 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.

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.

And she now has another young child, a six-year-old boy named Phoenix, whose life would become immediately imperilled.

Experts believe Paris remains a very clear and present danger, not least to those closest to him.

This, surely, is the ultimate test of a parent's love. Could you forgive a child who murdered another of your children? Could you continue to love that child? Could you bear the thought of that child being freed to potentially do it all over again?

As the father of four children, I honestly don't know the answers to these questions. I'm not sure any parent does until it happens to them. But I was fascinated to speak to Paris and Charity, to try to comprehend the reality of their situation.

I met Charity outside Ferguson Unit, a grim, forbidding state prison in Texas where Paris is being held. She's an intelligent, personable 45-year-old woman. But her eyes and demeanour suggest an ongoing anxiety that won't go away – both from the agony of what happened and what may happen in the future.

I asked her what to expect from her son when I went inside.

'He's human,' she replied. 'He'll be nice, personable, polite. Paris is very charming when he wants to be. I mean, he's a psychopath.'

I walked into the room where the interview would take place and found a dark, walled cage.

The armed prison guard was firm: 'You have one hour from the moment you start, not a minute more. I'll go get Paris now.'

A few seconds later, I saw a slightly built young man walk into the cage wearing a white boiler suit and stride towards me.

He was about 5ft 10in, slim and athletic, with his head shaved bald, a pronounced Adam's apple, thick black eyebrows, and a large silver chain around his neck.

'Hello everyone,' he announced. 'Since this is going to be done for ITV, would you like me to speak in the Queen's English?' He smirked as he said this. My mind raced back to a conversation I had the night before with two of America's top criminal profilers.

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

'Will he try to play me?' I asked Mark Safarik, a former FBI agent of 30 years.

'Of course he will,' replied Mark. 'For him, it's a game. He has an hour to get one over you and he's smart enough to try to do it, and narcissistic enough to think he's smarter than you.'

Dr Casey Jordan, a specialist in criminal behaviour, added a note of caution: 'He's researched you, I guarantee it, and he'll be sizing you up faster than you're sizing him up.'

I stared at Paris Bennett, and he stared at me. Then he smirked again. I found him immediately unnerving. As we exchanged small talk while our cameras were set, he spoke in a sharp, articulate manner, but his eyes never left mine.

Unlike every other killer I have met, he exuded supremely arrogant self-confidence.

'Why are you doing this interview?' I asked him once we started. 'To show people that I am not a monster or villain,'

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