The best thing about killing someone is the way their expression changes, as they realise the seriousness of their predicament. Take the youth I executed last night, for example.

We passed each other on the towpath of the canal near my home in Docklands, beneath a road bridge where the sodium lamps buzz and falter. I made the mistake of glancing at his face as he approached, so he scowled and occupied the centre of the thoroughfare, challenging me to skirt him or show some braggadocio. At first, I took the former option. Then I turned to meet his stare and hold it, until he wheeled around, clenching his fists.

“I’m terribly sorry to point this out,” I said, in the most plummy voice I could muster, “but your trousers seem rather too big for you.”

It was true. The gangsta-rap jeans were slung so wide and low they were practically falling off. Had I chosen to run at this point, he could not have given chase without tripping over his gnome’s crotch. For a moment, I found myself marvelling at the inefficacy of street-wear that would pin one’s knees together at speed; could he have a pair of quick-release, emergency braces hidden beneath the jacket, I wondered?

There was no time to ask. He evidently found uninvited looks and polite sartorial advice to be grounds for assault, and launched a slab-like fist at my head.

Too slow. I sidestepped the blow and used my left hand to pull the attacking arm forward, adding to its momentum and throwing the youth off balance. At the same time, I drove the nine-inch stiletto in my right hand directly through his heart.

No doubt you will form a moral judgement of this incident. However, before you do, let’s be clear about one thing: on my part, it was self-defence.

Admittedly, I knew the youth’s name, I knew where he lived and I knew when he was likely to walk here alone. I also suspected he would react with extreme violence to the kind of behaviour you or I would find inconsequential. I had even prepared a dossier of his criminal activities – grievous bodily harm, robbery, drug-pushing, terrorising the elderly - to plant on the body and send to the press. These were the premeditated circumstances of his death.

Yet, in the final analysis, had he acted like any normal person and resisted the urge to harass me, I would have permitted him to live. My actions on the towpath that night were entirely civil, while his reactions produced a crisis in which either I would end up savaged or he would end up dead.

The blade sunk quickly into the youth’s chest until the hilt slammed into his ribs. He tried feebly to bring his arms into a grapple but the pain took over immediately. I savoured the moment as his expression of malevolence turned to one of uncomprehending terror, then pushed him off the knife onto the tarmac.

There was very little time to spare before he lost consciousness, so I knelt down beside him and brought my lips close to his ear:

“This is for all of your victims, Joel,” I said. “You will never hurt anyone again, and the world is now a better place. Do you understand?” His head lolled towards me, blood seeping from the side of the mouth and down the chin. He was fading, but he could hear me all right. “Before you go, I want you to know that I discriminate against nothing but evil,” I said. “And you’re evil, Joel. Shall I tell you how I know?” The mouth opened and closed slightly. Blood gobbed from his chest like pond water from the penis of a garden cherub. “I know because you enjoy the suffering of others,” I said. “I’ve watched you laughing at their misery. I’ve taken photographs of some of the pain you’ve caused - just a fraction of it, probably. And I’m here to tell you that you’re part of a dying breed. There are very few evil people in the world, just very many who disagree over what is good. That’s why I’m such an optimist.”

I had more to say, but a movement from the canal suddenly caught my eye and I looked up to see a swan glide in silence to within a few feet of us. It stopped and looked at me quizzically, as if to enquire whether I might be dropping bread in the water today. The beauty of it stunned me for a moment, and I felt an overwhelming desire to apologise, not just for the ugliness of the scene, but for everything - the pollution, the fishing hooks that stole away too many cygnets, the orange street lamps that made it difficult to tell between night and day.

Most of all I wanted to apologise for human cruelty - just a few days earlier, I had seen Joel pouring paraffin onto a swan’s nest at the reserve further down the canal, and setting it on fire. I was too far away to stop the attack, but on reaching the scene had found him laughing with friends as screaming chicks tried to douse themselves in the stagnant water, too young to remain afloat.

Maybe last night's bird was from a different family. It peered at me for several minutes, unquestioning, and though I should have dashed from the scene I peered back, frustrated by its lack of exultation in what I had achieved. “I got the bastard,” I said quietly. “Look, you’re free. You’ve no need to be scared any more.” For some reason I sobbed a little, though no tears came.

When I tried to drag Joel’s body into the light, the swan started at my sudden movement and withdrew. It had obviously decided in any case that I was too occupied in human affairs to be of any help tonight. It turned gently and slid beneath the bridge, looking around for other sources of food or warmth or danger. And I watched it paddle silently into the distance, longing to be part of a world in which everything could be reduced to nothing more than sensation and instinct.