A vigilante was caught in a "web of lies" when he killed a 48-year-old man by shooting him "point blank" in the face, a Melbourne court has been told.

Daniel Duhovic, 34, has admitted murdering Bacchus Marsh father-of-five Paul Hogan, 48, on May 24, 2016.

The Supreme Court of Victoria was told on Friday Duhovic believed Hogan was raping and beating one of his female friends.

Duhovic and the woman, who has not been named for legal reasons, exchanged 216 text messages in the day leading up to the murder, as she repeatedly encouraged Duhovic to kill "it", the court was told.

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Prosecutor Jeremy McWilliams told the Supreme Court of Victoria on Friday the woman met both men online and had started a relationship with Hogan.

She told Duhovic via SMS that Logan had beaten, repeatedly raped and slapped her, and had also threatened to rape and kill Duhovic's young daughter, Mr McWilliams said.

Duhovic drove to Logan's house to pick her up, and saw the victim arrive in his van, the prosecutor added.

Duhovic walked straight up to the drivers' side of the van and "without speaking, shot Mr Hogan in the face at point blank range", McWilliams said.

A passenger sitting in the car was also injured.

A victim impact statement from Hogan's mother, who was present in court, was read out in which she described her loss like a "cannon" through her chest.

"I live in the past or the future as living in the moment is too painful," Mrs Hogan said in her statement.

"Paul didn't just die, he was stolen from me."

Defence barrister Philip Dunn QC told the court Duhovic, who practises Buddhism, had a "rigid moral code" exacerbated by an acquired brain injury from a road accident in his youth.

Duhovic had been barraged with messages of a horrific nature before the murder and had been caught up in the woman's "web of lies", making a "terrible error of judgment", Dunn said.

Justice Jane Dixon said the court needed to send a message that vigilantism and "taking the law into your own hands" was not acceptable.

Duhovic will be sentenced at a future date.