Marc Robertson, 43, had been friends with Aaron Grant before the 28-year-old started dating the mother of his four children.
There was animosity between the pair and another friend, Steve Barns, who Robertson believed had impregnated his new girlfriend.
The three men were inside Mr Barns' Seymour home in the early hours of March 29, 2025, when Mr Grant was killed.
Robertson had gone to the property armed with a large knife but he did not immediately use the weapon.
Instead, he ranted at the two men and claimed they had taken everything from him and he had nothing.
Robertson then called the mother of his children either a junkie or a whore, triggering her new boyfriend to launch himself at Robertson.
The two men grabbed each other and were grappling when the knife Robertson was still holding went into Mr Grant's chest.
CCTV captured Mr Grant crying out for help while Robertson fled out the front door, saying, "I f***ing warned you".
Paramedics tried to revive Mr Grant but he died on the way to hospital about 4.20am.
Robertson handed himself into police, telling officers he had been defending himself because he was "scared shitless" of Mr Grant.
He was originally charged with murder but pleaded guilty to manslaughter in the Victorian Supreme Court on Thursday.
Mr Grant's brother Travis told the court no words could describe the pain he was experiencing.
"There are times when I still struggle to accept he is gone and I will never see him again," Travis Grant said as he wiped away tears.
The victim's girlfriend and the mother of Robertson's children - Nicole Morrell - said the circumstances of Mr Grant's death did not feel like real life.
"How am I supposed to explain this to a child when I don't understand it myself?" she said.
Defence barrister Belinda Franjic said Robertson was abusing methamphetamine, had no fixed address and was emotionally troubled before the manslaughter.
He did not intend to stab or kill Mr Grant but he accepted responsibility through his guilty plea, the barrister said.
Robertson was genuinely remorseful and he had reasonable prospects of rehabilitation if he could remain abstinent from drugs, Ms Franjic said.
Crown prosecutor Jonathan Manning accepted that was the case and also conceded Robertson had not intended to stab Mr Grant.
But he noted Robertson still made the decision to arm himself with a weapon prior to the confrontation.
Robertson, who was dressed in a grey tracksuit with a short mullet and two plaits, remained quiet throughout his plea hearing.
He will be sentenced by Justice Amanda Fox at a later date.
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