The 31-year-old former-police officer calmly stepped into the witness box to testify at his NSW Supreme Court double murder trial on Thursday.
He was questioned about the events before he fatally shot couple Jesse Baird and Luke Davies in their urban terrace in February 2024.
As the only witness to the deaths, the former police officer has denied entering their Sydney home in February 2024 with an intention to kill.
He was asked about an incident in August 2023 where Mr Baird woke at night to find Lamarre-Condon with his phone and wallet in his darkened bedroom.
The 31-year-old told defence barrister John Stratton SC he had been invited over and a key to the home had been left in the letterbox.
He claimed he wanted to go in and check Mr Baird's phone to see if he had been chatting to other men.
The pair had broken up in April after the television presenter admitted he did not want a long-term relationship, unlike Lamarre-Condon, the jury heard.
After Mr Baird woke up, Lamarre-Condon said he panicked and ran, throwing the phone and wallet behind him as Mr Baird chased him down the street.
"It was stupid and I admit it was stupid," he told the court.
Mr Baird did not know it was him, and the pair recommenced a sexual relationship soon after, the jury heard.
The former-police officer had to be told multiple times to slow down as he rattled away his answers from the witness box.
"Sorry, I'm a bit nervous," he told Mr Stratton.
Their relationship only lasted until November when Lamarre-Condon faked a story on Instagram suggesting Mr Baird had sent him flowers.
He hadn't, he admitted to the jury, saying he had only created the post to test whether Mr Baird was seeing other people.
While at first he denied the post had anything to do with Mr Baird, he said he "completely owned up to it" 10 hours later.
Lamarre-Condon also denied he had gone to a Sydney hotel in May 2023 and used his police authority to view CCTV of Mr Baird there with another man.
He claimed he had been invited to the hotel by the TV presenter and only viewed the CCTV footage after being told by another officer that there had been a possible assault.
Lamarre-Condon began his testimony being asked about trauma he experienced as a police officer.
He described seeing a man shot in the head with a shotgun, an Uber Eats driver who had his head run over by a truck, and a man pushed from his balcony to be impaled by a metal pole.
His voice started cracking as he described a male police officer who died by suicide at Surry Hills police station.
He also recounted bullying at the hands of his superior officers, including one who said police should be able to push gays off cliffs like in the old days.
In October 2023, he described trying to end his own life in a Sydney national park but ended up walking away after it was unsuccessful.
Lamarre-Condon will continue his testimony on Friday.
He is set to allege he entered the terrace disguised as an Uber Eats driver and carrying his police-issued firearm in February 2024 to retrieve his belongings.
He has pleaded not guilty to two charges of murder, claiming the men were killed in self-defence.
Prosecutors allege Lamarre-Condon killed Mr Baird "execution-style" at close range through a plan borne of a jealous obsession, before shooting Mr Baird's new partner Mr Davies to eliminate a witness.
Days after their deaths, he led police to a rural monastery where the bodies of the two men, both aged in their 20s, were found in surfboard bags.
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