Four weeks ago I saw a murder on the internet. There isn't a punchline to this; it is not an intriguing play on words. Four weeks ago someone on a chatboard posted a link, with the exhortation: “See if you can keep your breakfast down after watching this! I couldn't!”

Since “See if you can keep your breakfast down after watching this!” is, as one poster pointed out later, the kind of comment that, in the 21st century, precedes a link to a very fat woman trying to get out of a very small car or - if the chatboard is really bitchy - that shot where Mischa Barton is mixing Lacroix and Chanel very badly, quite a few of us clicked on the link.

Instead, it took us to some footage shot on a mobile phone, in some bland, murky woodland. It appears to be early summer. Fifteen feet away there's a man on the ground. It's immediately clear that a great many terrible things have happened to him quite recently, and that he will die very, very soon.

The point of writing about this is that I have not really felt the same since I saw the murder, so I am not going to describe things in great detail - even though it is the details in watching someone die that are the most awful, and fascinating, and that rattle you the most.

Of the non-gory things, it is the man's trousers - grey, slightly worn but ironed; the kind that a poor, proud man would wear if he were going to the bank, say, or visiting more well-to-do friends - that were the most upsetting. He had dressed in great calm, and great order. He was now dying in unimaginable disorder and distress.

I do have to tell you that the man was being tortured - and not torture as shown on television dramas or films, which often looks like an aerobics session with a particularly strict personal trainer. One where you just have to “work through the burn” for a few minutes, like Madonna, before effecting an exciting escape. Two similar-looking teenagers were gathered around the man, and their torture was about brutally killing someone very slowly.

The footage is nearly seven minutes long. I stopped watching after 1.47. I felt physically different - very very high, in a bad way, as if I were going to pass out. I was also, with sudden irrationality, worried that the footage might in some way damage my computer, which I turned off, then unplugged, then covered with a cloth.

I think really that that is what I would have liked to do with my brain, but I couldn't. I still wasn't really sure what I'd seen. A large part of me was working on the hopeful premise that it was a very convincing drama project by some students - the kind of thing that was about to become a big viral hit, and about which the Daily Mail would become enjoyably enraged.

Simultaneously, I was telling myself that it was probably a revenge attack - that this man had attacked a lover, killed a child, and although his murder was awful, in a world of almost infinite sorrow it was not the unconscionably profane insult to humanity that it first appeared to be. I was using the thought of torturous retribution as a comfort.

At 3pm, doing the school run, I walked past the zebra crossings and recycling boxes, thinking what a surreal, inappropriate thing it was to be a mother of two, in a pair of bourgeois Ugg boots, going to pick up her children from school while thinking of a man being murdered in a wood.

Of course, it did occur to me that for whole generations - whole populations - walking down a street thinking of murder and death is absolutely commonplace. I could see why my granddad - in common with most men returning from the front - never talked about what had happened. I'd always thought that it was because they didn't want to say “I've killed a man” or “I saw a man being killed”, as the simple immensity of the fact would be upsetting. I realised now that it wasn't the simple, enormous facts that were upsetting but, as I mentioned before, the details, instead.

Any follow-up statement to “I killed a man” would involve the unexpected, quiet, horrible sounds; the sudden crash course in the structure of the skull; the slowness and then the quickness of blood. Best not to make the initial pronouncement in the first place.

By the time I got back home - on a walk during which I held the girls' hands far more tightly than usual - everyone on the messageboard was in uproar. Ric had found out more about the footage, and posted a Wikipedia link on the subject. The murder really was a murder - and not a drama project after all.

It happened in 2007, as part of a summer-long spree in which 21 people were murdered in Dnipropetrovsk, Ukraine. The trial is still going on. With possibly the biggest and most immediate sadness I have ever felt in my life, one penultimate sentence noted that most of the victims were vulnerable people - vagrants, the elderly, a pregnant woman, children. There was no comforting aspect of revenge.

And now, the additional nauseous business of the subconscious - for one unstoppable, white-light second - reimagining it all with children.

I don't want to overstate the whole thing, or be too dramatic. I had two subsequent nights during which getting to sleep was quite difficult, and I had to climb into my youngest child's bed and wrap myself right round her while pints of anxiety sat, like bad alcohol, in my guts. But it hasn't driven me insane, or made me question my world view. I am still an essentially shallow optimist. I am not damaged.

What I am, however, is host to something that will never leave. It made me realise that you should take great care in what you choose - often in a cavalier moment - to place in your memory, because some things will sit there for ever, like a bad seed; like a shadow on the moon; like a crow on a fence in a dream.

A very tiny part of me now, and will always, consist of an elderly man dying in a wood in Ukraine.

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