The 31-year-old has pleaded not guilty to murdering Jesse Baird and Luke Davies in a Sydney terrace in February 2024, arguing they were accidentally shot during separate struggles.
He told a NSW Supreme Court jury on Friday he went to Mr Baird's house on the morning of February 19 to retrieve belongings he left there during their relationship.
"It was going to be a normal day," he said.
Lamarre-Condon said plans went awry when he was unexpectedly confronted by his ex, shooting Mr Baird twice after he was lunged at with a knife.
The former police officer said he thought his ex-boyfriend was still alive, so he picked up a phone from the kitchen to call triple-zero before thinking he could flag down a passing ambulance.
Lamarre-Condon said he planned on calling authorities but was restrained by a panic attack.
"It felt like an out-of-body experience ... I was floating above my body and looking down at myself," he told the jury.
"I went to the couch, laid down ... and passed out for a period of time that could have been five minutes, could have been 40 minutes."
The Crown alleges Lamarre-Condon waited for Mr Baird's housemates to leave so he could kill his ex, but the 31-year-old said he became distressed after trying to revive Mr Baird.
"I remember walking over towards Jesse ... (and saying) 'please get up, please don't be dead'," he said.
Before entering the Paddington house, Lamarre-Condon first went to a cafe and saw an Uber Eats motorcycle outside.
He said it sparked an idea of disguising himself as a delivery driver so Mr Baird wouldn't accuse him of harassment.
Lamarre-Condon said he had the helmet on with his service-issued pistol hidden in its holster under his arm when he was spotted by Mr Baird, who he says was holding a knife while yelling "we're getting robbed".
"He couldn't see that it was me because the visor was down," Lamarre-Condon said.
"He lunged at me."
He said the pair struggled over the knife, asking Mr Baird multiple times to drop it, before the other man managed to pin him to a wall with the knife near his neck.
"This struggle felt like an eternity," Lamarre-Condon said.
"I thought 'shit, if he drives that into my throat, I'm going to be dead'."
He told the court "police mode" kicked in and he upholstered his gun, pointed it at Mr Baird's shoulder, and fired in a bid to incapacitate him.
Mr Baird grabbed the front of the gun and yanked it forward and the pair fell to the ground when Lamarre-Condon accidentally discharged the weapon a second time, he said.
"It was my fault," he told the court while wiping his eyes.
"I'm responsible for his death, but it wasn't intentional."
Lamarre-Condon's offer to plead guilty to manslaughter was rejected by prosecutors.
As he moved towards the front door, he said he was confronted by the man he now knows to be Mr Davies holding the gun which he "must have picked up from the living room".
"It was pointed towards me at my chest and he said 'I'm going to f***ing shoot you'," Lamarre-Condon testified.
He said he had his hands up and asked Mr Davies not to shoot him, but the other man raised the gun to his head and swore at him again.
The police training kicked in again, he said, and he tried to direct the muzzle of the gun away from himself and back towards Mr Davies as they struggled.
Mr Davies must have pulled the trigger because the gun went off and a bullet became lodged in the doorway, Lamarre-Condon told the jury.
Prosecutors allege Lamarre-Condon was motivated by jealous obsession when he shot Mr Baird execution style and then killed Mr Davies through a door to eliminate a witness.
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