A police marksman wept today as he described how he had no choice but to shoot gunman barrister Mark Saunders dead.
The Metropolitan Police officer, known only as Alpha Zulu 7 (AZ7), said he feared Saunders, 32, would kill him.
He told Westminster Coroner's Court how relieved he was when the gunman began lowering the weapon, only to move it upwards again.
But seconds later AZ7 said he fired his MP5 carbine to protect himself as he stared down the barrel of the 12-bore shotgun.
Struggling to maintain his composure, the officer said: "I saw the top of the barrel, the top of the gun starting to descend towards us and I continued to scream at him.
"I thought: 'He is actually going to shoot me'. So I started squeezing the trigger, but it seemed at that moment the barrel went back up again.
"I was so relieved. I released the pressure on my trigger. I wanted to give him every possible chance to drop the weapon, only there is only so far I can go."
Nicholas Hilliard QC, for the inquest, said: "Had he moved the gun though your line, through the position where you were?"
The officer replied: "In my mind, if he wanted to pull the trigger at that point I would have been too late. I would be dead."
The CO19 officer described how he was stationed on the roof of a Bywater Street property that backed on to Mr Saunders' £2.2 million Markham Square home in Chelsea.
He said Mr Saunders looked unsteady on his feet and could be seen swigging from a large glass of wine before smashing his kitchen window with the butt of his shotgun.
Coroner Dr Paul Knapman asked the officer, who gave evidence from behind screens, if he wanted to sit down as he cried and sniffed loudly.
The officer said that in the final seconds of the confrontation he hoped the gunman would drop his Beretta 12-bore out of the kitchen window into the garden below.
The inquest heard members of the public, including a woman with a child in her arms, were trapped in a house next door to the siege on the evening of May 6, 2008.
Mr Hilliard said: "Can you explain why - if it was not absolutely necessary to shoot him the first time?"
AZ7 replied: "I am not saying it was not absolutely necessary to shoot him the first time. Had he pulled the trigger it would have been too late and I may not be here to give evidence.
"What I am saying is I missed that opportunity because his action had beaten my reaction.
"He raised the barrel and again I was relieved he did not shoot me and I was relieved I did not have to shoot him.
"At the point I pulled the trigger, I believed he had already fired. Obviously the barrel came down a second time and as this happened I did not feel like I had much room for error.
"He looked more comfortable and his initial movement with the gun was to settle into his stance.
"As it came down the second time I could see the nostrils pointing at me, the end of the barrel, and I decided to fire my weapon for my protection and the protection of two guys to my right.
"At the point of firing I thought he had just fired it. I thought he had hit me.
"I believed at that point, when I was squeezing the trigger, he was about to fire.
"As I squeezed the trigger I do not know why I believed he had fired at me. There was lots of noise, lots of smoke."
Referring to the two moments when Mr Saunders lowered his gun, Mr Hilliard said: "The difference between the two times is to give him a second chance?"
AZ7 replied: "I did not want to kill anyone."
Mr Hilliard added: "Is that the reason you did not fire the first time, because you wanted to give him an extra chance?"
AZ7 replied: "I wanted to give him as much chance as he needs.
"As I said, his body language suggested to me, his initial movement with the gun when it went back up, he was not surrendering.
"He was just settling into a better shooting stance. I do not know how long this took. It was milliseconds."
Under cross examination by Patrick Gibbs QC, who represents the dead man's widow Elizabeth, the officer said police did not have the "luxury" of simply backing off.
He said: "I think what you are suggesting is a bit of a luxury for someone in our position, with our responsibility.
"I had seen the stranded occupants of next door. A woman with a child in her arms.
"In my mind it would have been very easy for Mr Saunders, without explanation or debate, to fire rounds into windows next door."
The officer added: "To go away, go home, would have been a luxury."
Earlier another police marksman known only as Alpha Zulu 8 (AZ8) told the inquest jury that he regretted not shooting Mr Saunders earlier.
He said he had been left "kicking himself" because he pulled the trigger "too late".
The inquest heard the officer was on the point of shooting as the barrister lowered his shotgun but changed his mind when it jerked upwards again.
Seconds later, as he crouched below Mr Saunders, he fired twice as the barrister dipped the end of his 12-bore shotgun towards him a second time.
The inquest heard the officer was left exposed in the conservatory of a property backing on to Mr Saunders' Chelsea home as he installed a spotlight.
After he switched on the battery-powered halogen light, AZ8 said the gunman turned towards him and a second officer, AZ9, as "events overtook us".
The officer moved into a kneeling position and aimed his MP5 carbine as colleagues shouted and screamed at Mr Saunders to put his weapon down.
Asked why he did not shoot Mr Saunders the first time, the officer said: "When he brought it down the first time I was virtually at that point.
"I took my breath, checked my sight picture and was just about squeezing one off.
"I was waiting for it, then he brought it back up. I took a lung full of air again and eased off on the trigger.
"As it came down again it crossed my line and I let off the first round."
AZ8 told the jury he said to himself "enough is enough" as Mr Saunders brought the gun down a second time, possibly with his finger on the trigger.
He added: "It just got past that point of no return. Like you say, I have seen the film, and I am kicking myself because I feel I left it too late.
"I should have taken the shot the first time he brought it down. I only had a nanosecond to make that decision."
AZ8 was referring to footage from a police helicopter which showed Mr Saunders' actions.
Mr Saunders suffered catastrophic injuries when he was shot five times in the head and chest by police officers surrounding his home on the evening of May 6, 2008.
The inquest has heard the alcoholic family law specialist sparked the siege by firing his Beretta weapon through a kitchen window after a lonely drinking binge.
The inquest heard AZ8 was mistakenly told Mr Saunders had fired at members of the public and police after a domestic row in which his wife had "run off".
It also emerged the police helicopter pilot was ordered to descend to drown out the noise of AZ8 and his colleague as they forced their way inside 3 Bywater Street.
The jury has been told the move to erect spotlights came despite the decision of a senior officer that it would be too dangerous.