Ron Fanelli was a poker player: loud, brash, rightwing, ex-US navy. Victoria Coren liked him. Then last week she learned that he had confessed to killing bar girl Wanphen Pienjai in Thailand and chopping up her body

'Seemed like a normal guy to me." That's what the neighbours often say, isn't it? "He was pretty quiet. Kept himself to himself." Last week, I walked past the house in Brighton where police are digging for victims of the serial killer Peter Tobin. I felt sorry for the people next door. It's bad enough to be woken by drills when someone's getting cable TV.

I wondered if the neighbours had lived there when Tobin did, in the 1980s, and were now getting the opportunity to tell people he seemed like a normal guy to them.

My old poker friend Ron Fanelli never seemed "pretty quiet". He was a noisy guy, opinionated. He broke all the rules of London poker etiquette by turning up at the casino, six or seven years ago, and sounding off right from the start. But I forgave him all the noise, because he was American. He was funny. I liked him.

When I hosted a series on the Poker Channel, a niche chatshow with poker players as guests, I invited Ron to take part in several episodes because he was entertaining and outspoken. One episode had the theme of "table manners": what is and is not acceptable behaviour in a poker game. My opening question was: "Ron, what is the worst thing you've ever done?"

Ron replied: "Me? I'm an angel. I've never done anything bad. Well, I guess I've made a few people cry. I don't like getting unlucky. There was that time I told everyone at the table I hoped they'd die of cancer. Other than that, I've never done anything bad."

And he laughed. The other guests laughed.

Last Monday, Ron confessed to murdering a prostitute in Thailand, chopping up her body and disposing of it in a suitcase.

It is a staple of broadcast news, when a killer is identified, to ask personal acquaintances for clues from the past. The public is eager to help. Everyone has a story to sell about Raoul Moat. They saw it in him. They never saw it in him.

I was always sceptical of anecdotes about "strange eyes" or "a nervous manner". All meaningless hindsight. Of course you can't tell if the man next door is a potential murderer. If you could, you'd move house.

When I first heard that Ron had been arrested in Thailand for the brutal stabbing of Wanphen Pienjai, an employee of the Sweetheart Bar in Phuket, I assumed he had been fitted up. I was terrified for him. I automatically supposed that this had been a scandalous murder, there was pressure on local police to make an arrest and what better scapegoat than a noisy American immigrant who visited prostitutes? A good one to lock up, close the files and draw a line under the case.

When I heard he had confessed, I thought he must have been coerced into it. I knew he'd been offered a sick deal whereby he would be executed if found guilty, unless he confessed and accepted life imprisonment. If I were offered something like that in Thailand, I thought, I would probably confess to anything. And then I would sit in jail and wait to be rescued. There is no way this man, whom I knew and liked, had actually done it.

But then the police found the knife in Ron's house. He gave them the shorts he was wearing at the time of the murder. They took away his mattress. He pleaded that he had been drunk at the time. He said it was an accident. He explained how he had put Wanphen's dismembered remains into a suitcase, balanced it on the front of his motorbike, and ridden off to dump the poor, lost girl along the Chao Fa Thani road.

And then the thinking starts. You can't help looking back on every encounter and wondering what it was you were supposed to notice. And then you notice things.

We called him "the Mad Yank". That was his poker nickname. He was temperamental, but you don't give someone a "mad" nickname if you mean it. It was just a joke. He loved it. I think the name might actually have been his own idea to begin with.

I had an argument with him once, when he first started dating Thai girls. He told me that western women were "strident feminists. Bossy and demanding. Asian women are docile, they understand what men want."

I told him not to be so bloody silly. I told him that women are the same the world over, and not to be fooled by the clever tricks of one who might be angling for marriage. (He did end up marrying a Thai woman. They had a child, in Thailand, and a few months later she left him.)

Most poker players are lovable old sexists. Ron was a rightwing American who had served in the navy. I didn't take the argument seriously. Looking back now, it takes on a sinister tone.

I have a photo of myself with the Mad Yank, from 2004. He was having a great time then: winning at poker, popular on internet forums, appearing on chatshows. He played it up, growing an exaggerated moustache and wearing sunglasses. I thought he looked funny in the picture. Staring back at it now, he looks like a killer.

If it were revealed that Ron had, for some reason, invented the whole detailed confession and was not guilty, he would stop looking like a killer. But that is what your brain does. Last Sunday night, I assumed Ron had been fitted up. Twenty four hours later, I had rethought everything in a new light, filled in the gaps, written a murder story in my head, and it made perfect sense.

By 2006, we all knew Ron had run out of money. His hot poker streak had fizzled out; he was kipping on friends' floors and borrowing money. His pride was dented. He was no longer the big success story. He looked for occasional work as a croupier, dealing cards to people whose money he had once won. When he met a "docile" Thai girl, her deference boosted his damaged ego. The obvious move was to follow her to Thailand, living in "paradise" (as he described it on his blog) where everything was cheap and he felt important again.

But she left him. He married a different Thai girl, had a child, then she left him too. Ron was broke but still gambling. He was short-tempered. He demanded obedience. These women had no need of a difficult, impoverished husband in their own homeland. That wasn't the deal. They grew tired of deferring, with only that in return. If he couldn't be an old-fashioned provider, why be an old-fashioned housewife?

When his wife left, Ron felt fooled and betrayed. The women who were supposed to make him feel important had made him feel stupid. It had all been a con. The anger and shame ran deep. He still had little money and no job. At this point, Ron started to use prostitutes. It was a power thing. He used more and more of them. After a while, hiring them for sex was not enough to make Ron feel powerful. He made them do sicker and kinkier things.

He didn't know what had gone wrong. He was a clever, articulate, former military man. He used to win money in glamorous poker tournaments. He had been an alpha male. Now, here he was, stuck in a foreign country where he had chased a woman who left him; another had taken his child away; he was unemployed, skint and using prostitutes. Ron knew he was a man to be reckoned with, even if nobody else could see it. Even if his wife couldn't see it.

No amount of hookers, no manner of kinky activity, could fill the hole where Ron's self-esteem used to be. He knew, now, that the women's obliging, flattering manners were just a pretence. That's just what women do to get what they want. He despised them for it. What he didn't understand was that the more he degraded and punished the girls, the worse he felt about himself. And the further he had to go to feel masterful again. It was a dark, twisted cycle.

One day, he picked up a beautiful girl from the Sweethearts Bar, round the back of the Club Med resort in Kata on the island of Phuket. He took her home. And when she reached for him, with her obliging smile, telling him how lucky she felt to find such a handsome customer, he was revolted by the lie. He saw, suddenly, all the women in the world, every last bitch taking him for a fool. Ron would not be taken for a fool. He snapped.

That is the story I have written for Ron in my head. I know he ran out of money. I know his wife left him. I know he posted on internet forums about all the kinky things he was up to with Thai hookers. I know he has confessed to this murder. I don't know anything else. I have no idea how he felt, or feels, about anything. There was a line in a news report from the Phuket Gazette that made me cry. It said:

"Asked if he were the same Ronald Fanelli who was a former professional poker player who gambled in tournaments under the name 'Mad Yank', he laughed briefly with a tone of bitter irony before replying, 'Yes, but that was a long time ago.'"

But it wasn't. It wasn't a long time ago.

The fee for this article has been donated to Victim Support