Marc Robertson, 43, has pleaded guilty to manslaughter for fatally stabbing his former partner's new boyfriend, Aaron Grant, 28, during a confrontation on March 29, 2025. Robertson admitted to killing Grant after an argument at a home in Seymour, Victoria, where he had gone armed with a knife.

According to court documents, Robertson had been friends with Grant before Grant began dating the mother of Robertson's four children. Animosity had developed between Robertson and Grant, and another friend, Steve Barns, who Robertson suspected of impregnating his new girlfriend. All three men were at Barns' home in the early hours of March 29, 2025.

Robertson arrived at the property with a large knife but initially engaged in a verbal altercation. He ranted at Grant and Barns, expressing that he felt he had lost everything. The situation escalated when Robertson made derogatory remarks about the mother of his children. This prompted Grant to attack Robertson, and the two began grappling.

During the struggle, the knife Robertson was holding entered Grant's chest. CCTV footage showed Grant crying out for help as Robertson fled the scene, reportedly saying, "I f***ing warned you." Paramedics attempted to revive Grant, but he died en route to the hospital around 4:20 a.m.

Robertson later surrendered to police, claiming he was defending himself and was "scared shitless" of Grant. Initially charged with murder, Robertson changed his plea to guilty for manslaughter in the Victorian Supreme Court on Thursday. He is awaiting sentencing.

Mr. Grant's brother, Travis, described the profound pain of his loss to the court. The victim's girlfriend and mother of Robertson's children, Nicole Morrell, expressed her difficulty in comprehending the circumstances of Grant's death and how to explain it to their child.

Robertson's defense attorney stated that prior to the incident, Robertson had been abusing methamphetamine, lacked a fixed address, and was emotionally distressed. The defense argued that Robertson did not intend to stab or kill Grant but accepted responsibility through his guilty plea. The prosecution acknowledged that Robertson did not intend to stab Grant but noted that he had armed himself with a weapon before the confrontation.

Robertson will be sentenced by Justice Amanda Fox at a later date.