Siege situations are the most frustrating a police officer can get involved in. You've got very little control over events so you can never really relax an inch, just in case something happens. But more often than not, nothing does. The suspect mulls over his actions, finally works out that he's trapped and that he's not going to get out of there except in handcuffs or a box, and eventually releases his hostages and simply walks out the door. It's frustrating because you want to be doing something to help end the situation, yet in most ways you're pretty much irrelevant to it.

On the day of the Haringey siege I remember it was hot. Stiflingly hot. We'd been on the scene about an hour and had the place completely surrounded when, without warning, our hostage taker had suddenly appeared in the front window, naked from the waist up, holding his gun. He was a big guy with the beginnings of a pot belly and a tattoo of an eagle straddling his chest. He'd shouted something from behind the glass, then opened the top part of the window and stuck his head out, shouting something else unintelligible. I was ten yards away behind a car on the street. Another officer was crouched down beside me. He was about fifteen years older than me and his name was Renfrew. I remember he got pensioned off a couple of years later after he got a glass in the face trying to break up a pub fight. Renfrew cursed the guy under his breath. You could tell he wanted to shoot him. Why not? The guy was just a waste-of-space dopehead who caused a lot more harm to the world than good. But Renfrew was a pro and, like a lot of coppers, he had one eye on the pension, so he was never going to do anything that might jeopardize his career. I was still a bit idealistic in those days. I didn't think about the pension. I thought about the wife and kids stuck in there with an unpredictable maniac.

I'd had an earpiece on. The chief superintendent spoke into it. Don't fire, he said. We're still negotiating. Keep him in your sights, but don't fire.

Then, just like that, our target had brought the gun up and pointed it wildly towards us. The chief superintendent hissed something else into my earpiece, but I didn't hear it. It looked like the suspect was going to pull the trigger. I knew he wouldn't hit me from where he was standing. I had good cover, and he looked too stoned to aim straight, but I was still nervous. And angry. This bastard was just showing off his power, knowing we'd have to stand there like lemons, hamstrung by our limited rules of engagement. That got me; it really did.

So I'd fired. Two shots from the Browning. Straight through the window and into his upper body. One of them got him in the heart, but the autopsy confirmed that either of the bullets would have been fatal on its own. He died instantly, I think. Certainly before anyone could administer first aid.

I was offered psychological counselling and I took it because I was told that if I didn't, it would look like I didn't care that I'd killed a man. It didn't do me much good, mainly because I genuinely didn't care that I'd killed him. In fact, I was quite pleased. He'd wanted to kill me and I'd got in there first. But of course I didn't tell the counsellor that. I told him I deeply regretted having to take a life, even if it was in the line of duty. I guessed that that was what he wanted to hear.

There was an inquest, and I was forced to give evidence. There was even talk about a criminal trial, especially when it was discovered that the gun he'd been holding was a replica. I was suspended for close to two months, although at least it was on full pay. On the second day of the inquest, I was leaving the building by a side door when I ran into the common-law wife and her brother. She spat in my face and called me a murderer while the brother punched me in the side of the head. A uniformed officer intervened before things went any further, but the incident taught me two things. One, never rely on the support of people you're trying to help. As politicians have often found out to their cost over the years, the hand that pats you on the back one day can just as easily grab you by the balls the next. And two, never rely on anyone else for support either. In this world, you've got to get used to the fact that, in the end, you're always on your own.

No blame was ever officially attached to me over the killing of thirty-three-year-old Darren John Reid (who, it turned out, had a grand total of twenty-nine convictions, including eleven for violence, four of which related to his missus), but it might as well have been. I was taken off any further firearms duties (and have been to this day); banned from keeping guns privately; and my path up the career ladder slowed down one hell of a lot over the next few years. Crime, it seems, only pays when you're a criminal.

I'm not a bad man, whatever those who like to sit in judgement may think. When I started out I really did believe I could make a difference. My sole motivation was to take the bad guys off the street and bring them to account for the crimes they'd committed. After the Reid shooting, I slowly stopped caring. I suppose I finally realized what all defence lawyers know: however well intentioned its designers may have been, the law in practice only serves to help the criminal, hinder the police, and ignore the victim.

Having got to a point when I was as cynical as that, it was only a matter of time before I fell in with the wrong company. The wrong company being, in my case, about as wrong as you can get, although when I first started doing business with Raymond Keen, one of North London 's more colourful entrepreneurs, I wasn't to know quite how far it would go.

I've had a business relationship with Raymond for about seven years now. At first it wasn't too serious; nothing like that ever is. Just a few tips here and there, a helpful advance warning of impending police action, a sale of the odd bit of dope that went missing from police custody. Small things, but like cancerous lumps, small things that inevitably grow bigger. I wasn't even that surprised when two years ago he asked me to kill a bent businessman who was refusing point blank to pay him the twenty-two grand he owed him. The businessman was a nasty piece of work. One of his sidelines was importing kiddie porn. Raymond offered me ten grand to get rid of him. 'It'll strike a blow for creditors everywhere,' he'd said, although I wasn't quite sure how many creditors would follow his example and write off their debts with that degree of permanence. But ten grand's a lot of money, especially when you're on a copper's wage and, once again, he wasn't the sort of bloke anyone was going to miss. So one night I waited for him outside the lock-up he used. When he came out and walked over to his car, I emerged from the shadows and followed. As he opened the door, I pushed the silencer against the back of his hairless head and pulled the trigger. One shot was all it needed, but I added a second for good measure. Pop pop. All over. And I was ten grand richer. It was very easy.

But three men dead in one go? Danny was right; there was going to be a lot of heat over this one, although Raymond, who was the instigator of the whole thing, didn't appear too worried that any of it would get back to him. But then Raymond wasn't really the worrying type – which, I suppose, in his line of business, is something of a plus.

It was getting late. I drained my wine, drank a glass of water from the tap so that I didn't dehydrate myself, and made my way to bed. Looking back now, I already had a bad feeling about the whole thing but I was trying hard not to admit it to myself. Raymond Keen had paid me forty grand for killing those men. It was a lot of money, even after Danny had got his 20 per cent cut. Enough to justify a lot of things.

But nothing like enough to justify what was to follow.