A Met firearms officer told an inquest he repeatedly told a father-of-three to drop his weapon before the man was fatally shot in east London.
But Richard Cottier, 41, continued to advance on police marksmen, a jury heard.
Mr Cottier was shot at a petrol station in Romford in the early hours of April 9, 2018 after telling officers he had taken a drug overdose, made threats and claimed to have a firearm.
An inquest into his death at Barking Town Hall previously heard from his long-term partner Melissa Cottier that his actions had been a “cry for help”.
On Thursday, one of the officers who shot Mr Cottier, told the court he “honestly believed” he was going to be killed.
The officer cannot be identified for legal reasons and was referred to as PW47.
Giving evidence, PW47 said he had been aware that a “firearms incident” had been declared just before 4.30am, and that Mr Cottier had been declared as “EMD” (emotionally or mentally distressed).
Although he and two other officers – known as F79 and KH13 – were initially told to go to Mr Cottier’s house, they were redirected and told to proceed “with urgency” to the Esso petrol station on Collier Row Road.
PW47 said he saw a man he identified as Mr Cottier “violently slamming” a gun against a vehicle on the petrol station forecourt and then “brandishing it at a member of the public”.
“Doing nothing was not an option… from what I saw I was very concerned,” he said.
Asked by coroner Nadia Persaud if he thought the weapon was “capable of lethal force”, PW47 replied: “Absolutely.”
“I got out of the passenger seat (and)… shouted as loud a challenge as possible of ‘armed police’,” he said.
“We pushed forward as a team to get Mr Cottier away from the public.
“He walked… towards myself and F79. I continually shouted to him to drop the gun, or words to that effect, but unfortunately Mr Cottier didn’t take action to drop the weapon.
“He pointed the weapon directly at myself and it was at this point I discharged the round.
“Very shortly after there was a second shot… from F79.”
The inquest was shown bodyworn footage of the incident recorded by PW47.
Stephen Simblet QC, representing Mr Cottier’s family, asked PW47 if he had considered using his Taser.
The officer replied: “I was head-on facing another firearm so it was not something I would consider at the time.
“It was a dynamic, quick, developing situation.
“At the time I honestly believed I was going to be killed.”
The inquest also heard from the officer known as KH13, who said he too believed there was a “real and imminent threat to life”.
“It felt like time slowed down,” he said.
Asked whether there was any point at which he doubted the gun was real, PW47 replied: “Absolutely not.”
The police watchdog said that a non-police firearm was recovered from the scene following the incident and that a ballistics expert had confirmed the weapon was a modified air rifle.
PW47 added that immediately after the shots were fired, he and F79 had tried to save Mr Cottier from the “catastrophic wound” on his neck.
“It was a case of saving his life,” he said. “We needed to do everything we could to achieve that if we could.”
PW47 left the Met in 2018 but had already applied for the transfer prior to the incident.
Jurors heard it had been the first time he had ever discharged his firearm.
The inquest continues.