Beau Lamarre-Condon, 31, also tried to cancel the man's house move after his February 2024 death and earlier told a confidant how he'd dispose of a body after a homicide, a Supreme Court jury was told.
The former NSW police officer has pleaded not guilty to the murders of his ex-partner Jesse Baird and Luke Davies, suggesting he acted in self-defence when the men died in separate struggles in Mr Baird's house.
A former friend and police colleague told the trial the accused killer was obsessive about Mr Baird's personal life.
"Beau went to Jesse's Paddington terrace and there was a cup of water on the bedside table," Sergeant Andrew Callus said on Monday.
"Jesse must be seeing someone else," Lamarre-Condon allegedly said.
Sgt Callus also recalled how his former colleague's attitude darkened when he found out Mr Baird, 26, and Mr Davies, 29, had started a relationship.
"You know what I'm like ... I hate losing, I can't let things go," Lamarre-Condon allegedly said.
After the two men died, a series of text messages were sent from Mr Baird's phone. Prosecutors alleged they were sent by Lamarre-Condon.
Daniel Hart attempted to contact Mr Davies on the day the pair died, having met up with him and Mr Baird the night before.
But he noticed his messages were not being delivered, so he tried messaging Mr Baird instead.
"Is your wife alive?" Mr Hart asked, referring to Mr Davies.
"Haha he did smash his phone screen Monday so idk if he's got that fixed yet (sic)," came the reply from Mr Baird's phone.
The next day, another man sent Mr Hart a screenshot of more messages from the same phone.
They purported to be Mr Baird trying to cancel a new home-sharing arrangement.
"A possible work opportunity has come up in North Sydney ... I might want to live closer to work (so) count me out," his intended housemate received in a message.
Earlier, Lamarre-Condon's former close friend Georgia Cook continued giving evidence about a dinner she had with the accused murderer.
"If you did kill someone, what would you do with their bodies?" Ms Cook said he asked her in May 2023.
When Ms Cook said she didn't know, the then-officer allegedly replied: "I would take their stuff, put it in a dumpster and I would take their bodies to bushland where no one would ever find them."
One of the officer's barristers put it to Ms Cook in cross-examination that she only included those details in a police statement after she heard them in media reporting on his arrest.
"When someone says something that scary or shocking, it's not something that leaves your mind," she replied.
Lamarre-Condon allegedly dumped the two men's bodies at a rural Greek monastery after unsuccessfully trying to sink them in a dam.
The former officer accepts he entered Mr Baird's Paddington house in February 2024 armed with his police pistol while heavily disguised.
But the defence case is he did not intend to kill his former partner or Mr Davies.
Ms Cook said the conversation caused an irreparable breakdown in her relationship with Lamarre-Condon, especially as he allegedly blackmailed her to stop her revealing his plans.
"What I'm about to tell you ... if you tell anyone else, I will go and tell people what I know about you," Lamarre-Condon allegedly warned her before discussing the disposal of bodies.
As well as contesting two murder charges, Lamarre-Condon denies breaking into Mr Baird's home in August 2023.
His barrister previously conceded his client did some "stupid things" but they did not rise to murder.
The trial continues.
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