Warning after 'unplanned' killing of ex's new flame

A killer could be out of jail within two years after he fatally stabbed his former partner's new boyfriend in a spontaneous and unplanned attack.

Marc Robertson, 43, looked relieved as Victorian Supreme Court Justice Amanda Fox handed down his sentence on Wednesday morning.

He pleaded guilty to the manslaughter of Aaron Grant, who was 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's 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.

Justice Fox described the offending as spontaneous and unplanned as she handed down the sentence of five years and six months behind bars.

But with time already served, Robertson will be eligible for parole in June 2028.

Justice Fox accepted Robertson did not intend to kill Mr Grant and did not plan to use the knife he took to the home.