But the decision to still steal a dirty old baseball cap after the brothers had been stabbed - one fatally - was almost breathtakingly callous, says the judge who will now sentence the robbers.

Chol Kur and a 17-year-old boy who cannot be named, were the instigators of an attack on the teenage brothers at a bus stop in Kings Park, in Melbourne's northwest in September 2019.

Maaka, 17, and Nate, 19, were on their way to the gym when a car pulled up and Kur and the other boy jumped out to "drill" them.

Kur demanded Nate's hat, a Philadelphia '76ers basketball team hat he'd had for years. He refused and they scuffled while the younger robber held Maaka in a headlock.

Then Joshua Horton jumped out of the car, put his hand on Maaka's back and stabbed him in the chest, the blade piercing his heart.

Nate was stabbed twice and suffered life threatening blood loss.

The trio ran back to the car and fled, but not before taking Nate's hat.

Horton was acquitted of murder but found guilty of manslaughter and intentionally causing serious injury. Kur and the other boy pleaded guilty to robbery.

The teen's lawyer Tim Marsh told a pre-sentence hearing on Wednesday that the object of the robbery as neither valuable nor significant in any way, it was a crime committed "for the sheer devilry of committing a robbery".

"I would concede at the outset the sheer pointlessness of this offending is stark and difficult to reconcile," he said.

"The fact the robbery was completed after the stabbing of both the victims ... is a factor which can only be described as callous and I completely concede that."

Justice Andrew Tinney agreed, saying it was almost breathtakingly so.

The teen has spent 18 months - nearly a 10th of his life - in custody, Mr Marsh said suggesting the boy would benefit from a period of transition back into the community.

"The capacity for young people to make ill-advised decisions in the moment ought not condemn them in the same way the ill-advised actions of adults condemn them," he said.

The Hakiwai family spent two days sharing their grief with the court.

"There are no words to describe how many lives the three of you have ruined in 16 seconds, including your own," Nate told the three men, through a statement read by his father Stirling Hakiwai on Tuesday.

Justice Tinney will hand down sentences in the case on July 2.

Australian Associated Press