The now 30-year-old was previously jailed for at least 21 years for the murder but faced a retrial after his first conviction was overturned on appeal.
"The attack upon the deceased was brutal and his killing was gratuitous and callous," said Justice Peter Garling in the NSW Supreme Court on Thursday.
He heard the second trial without a jury, finding Carbone had entered into an agreement with his cousin kickboxer Tony Bagnato to at least grievously injure Bradley Dillon.
In August 2014, the pair lured the 25-year-old into a Leichhardt underground car park where he was stabbed repeatedly from behind and shot three times as he fled.
The judge was unable to determine who stabbed Mr Dillon, but found Carbone engaged in a physical struggle with him.
He also found Bagnato brought a Glock pistol to the meeting and used it to shoot the victim.
He determined Carbone's role was subsidiary to that of his cousin, who fled to Thailand after the murder.
"However, the role of the offender was not one of being a mere bystander doing nothing at all," he said.
Having being part of a joint criminal enterprise to attack Mr Dillon and cause him serious harm, he was responsible for his cousin's actions.
"In other words, he is equally guilty of this brutal, callous and gratuitous murder."
Mr Dillon had been chasing repayment of money his sister lent to a member of the Saint Michael Fight Club.
Carbone was a former member and Bagnato a leader of the boxing and martial arts club. Both knew Mr Dillon from school.
The judge said there was no evidence to indicate Mr Dillon in any way initiated the violent attack.
He had been attempting to have money repaid to his sister and was seeking the intervention of others to help resolve the dispute.
Victim impact statements from his family made clear his death has caused significant harm and distress to them, the judge said.
His mother described his attackers as "spineless cowards", noting all her son's injuries were from behind.
One of his sisters, Kellie Dillon, said the family had suffered through seven years of relentless waves of grief.
The judge found Bagnato was a much more forceful character than his cousin, who was young and much less mature.
"The offender's personality was such that he would have found it difficult to resist taking part in the meeting and falling in with Tony Bagnato and his wishes, or perhaps what the offender perceived those wishes to be."
Justice Garling set a maximum term of 24 years, four years less than his original total sentence.
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