On 22 August Sharon Lopatka set out to look for someone to kill her. So she posted an Internet message to the discussion group alt.sex.necrophi lia: "Want to talk about torturing to death?"

Her message continued: "Hi my name is Gina. I was wondering if anyone out there would want to talk about the subject mentioned above with me. I kind of have a fascination with torturing till death ... of course, I can't speak about it with my friends or family. Would love to have an e-mail exchange with someone - If you're interested ... e-mail me at: Gina108@aol.com

"I hope you all don't think I'm strange or anything ... I just want to talk about it."

This disclaimer seems to have been a lie, just like the name she gave. She is certainly dead. Her body was found 10 days ago in a shallow grave outside the caravan where one of her e-mail correspondents, Robert Glass, lived. He has been charged with her murder. His lawyer claims he killed her by accident, during an episode of sex with strangulation.

She had taken a train from her home in suburban Maryland to Glass's caravan in rural North Carolina three days before. She told her husband Victor that she was visiting friends; she also left him a note saying: "If my body is never retrieved, don't worry. Know that I am at peace."

So far, it sounds like an everyday tale of American weirdness. What makes it exceptional is two things: the way that Lopatka's progress can still be traced across the Internet, and the curious, wrenching domesticity that such tracing reveals. She was herself married, and the daughter of an orthodox rabbi. Much of her participation on the Internet was as a rather desperate seller of home improvement tips: a kind of low-rent Jocasta Innes. One of the two small Web sites she ran started off like this:

"Home Decorating secrets seen in the posh homes from the New England states to the Hollywood homes can now be yours. Never published before! Quick easy ways to decorate your home. Thousands of decorators will be furious when they hear that we are giving away their professional trade secrets (unknown to all outside the industry). For the first time in print these secrets and tricks of the trade can now be yours.

"How to glamorise your walls without messy wallpaper or hiring expensive decorators.

"Easy sew and no sew home decor projects that anyone can do! Transform any room in your house into a decorator's showcase!"

This kind of greedy, resentful consumerism is familiar from thousands of small ads in the back of American magazines. But out on the wilder reaches of the Internet, Lopatka took it further.

Already fat, she advertised in a group whose fetish was cannibalistic sex, for someone to force-feed her until her weight reached 475 pounds. "I am not interested in e-mail correspondence or a phone feeding. What I would really like is the REAL thing! I am willing to be force-fed to meet my goal if necessary. I am also willing to relocate if that's what it takes to find the right feeder. I am hoping someone out there will help me out and share in the most erotic experience of their life.

"I don't want to break up any marriages," this post concluded, with her customary surrealism, "so if you're married, please don't respond to this post."

Robert Glass, who killed her, had been married. He had three children, and a wife who left him in April this year. She told the Washington Post that this was because he had completely lost interest in her. "The final straw came when the kids asked me why Daddy didn't love me any more."

He had a dead-end programming job for the local administration; he continued to attend it scrupulously during the three days that Sharon Lopatka stayed with him, and after he had buried her. The messages left on Lopatka's hard disk at her home had led the police to him very quickly. They found a fresh grave outside his caravan and arrested him at work. He said nothing when finally arrested.

Out on the alt.sex groups where the couple met, reactions to the story have been mixed. Some people see it as a terrible warning about the necessary boundaries between fantasy and reality. For others, the fantasy goes on. Someone signing themselves "Perro Loco" (Mad Dog) said he had corresponded with the killer. "He's a compassionate man," he wrote on 2 November. "But was unfortunately stupid for not insisting that Sharon ... Lady L bring her hard disk with her for him to wipe ... Not to mention that he should have taken her somewhere 'safer' to 'do' her."

Curious as to who could write such a message, I ran a search for all messages posted to Usenet discussion groups by this "Perro". There is a computer that records and indexes every one of the millions of words posted every day to Usenet. It shows him to be active almost everywhere people seek suffering and pain for sexual gratification. On 12 October, for example, he replied to an advertisement in alt.personals.bondage, which read, complete with typos and misspellings: "I have a fantasy of being kept completely naked, used for sex, feed well so that I'd gain weight, and then sacraficed and cooked. I'd love to hear what you'd like to do to me."

"Dear Piggy," replied Perro, "Depending on your gender ... we might be able to work something out ... feel free to drop me a line by e-mail."

All of this is so very much nastier, and more genuinely depraved, than anything most of us can imagine, let alone encounter in our everyday lives, that it is easy to feel that the Internet, or portions of it, are uniquely dangerous. And perhaps they are. I certainly wouldn't want my children to stumble on any of it.

Yet there are two ways in which the Internet is in fact less dangerous than real cities, because it offers so much less privacy to its users. Almost everything is recorded somewhere. There are exceptions to this rule. E-mail is generally private - though often hoarded by its recipients for the police to find afterwards. There is a sort of CB radio of the Internet, called IRC, where much of the nasty stuff goes on. IRC Channels such as pounds snuffsex (recommended by Perro Loco) flicker in and out of existence constantly, as near to anarchy as the Net nowadays gets. But most postings to Usenet and most Web sites are far more public than users realise.

Huge databases such as Altavista and DejaNews store almost everything on the Internet in accessible form. Others store millions of e-mail addresses and the corresponding phone listings; still others will print out a map showing how to get to any given address in the US, and all of these are free to casual users. If someone approaches you over the Internet, it is quite easy to find out a lot about them.

This is even more true in the various communities or tribes that cyberspace seems to generate. They are not real communities, but they do permit a lot of social interaction over a long period. This makes them a wonderful way to meet people, compared with some urban alternatives such as lonely hearts advertisements and bars. Despite all the cases of folie a deux, if you want to see how someone behaves over a long period and with many interlocutors, Internet conversations allow this quite easily. And they allow people to appear at their most attractive, with reflections, spontaneous flashes of wit, and playfulness.

This kind of recreational typing is tremendously infantilising. That is its charm, its power, and its danger. Most of us don't get to choose who we want to be after the age of about 11; yet we live in a culture where choice and perpetual reinvention of the self are held up as the blissful essence of celebrity.

On the Internet, the joke goes, no one knows that you are a dog - or a fat, miserable suburban wife. No one even knows whether you're serious about being killed and tortured. Perhaps you don't even know yourself