Raymond Harris, 27, died after being stabbed in the stomach when a group of men charged at another at Surfers Paradise in September 2020.
Jye Sebastian Webb-Italia, 22, was on Monday found guilty of murder for his role in the fatal stabbing that occurred in the busy Cavill Avenue tourist precinct about 9pm.
The same jury also found his brother Kyle Jack Webb, 28, guilty of manslaughter after deliberating for about five hours.
Another man on trial, Jarod James Miller, was found not guilty of both murder and manslaughter.
The brothers on Tuesday stood silently and still in Brisbane Supreme Court as they listened to victim impact statements before they were sentenced by Acting Justice Ann Lyons.
During the trial, the court was shown security camera footage of Mr Harris being stabbed during the confrontation while people visited a nearby convenience store and ice cream parlour.
The jury earlier heard Mr Harris had accused Webb of associating with a man known only as "Chaos", who had repeatedly robbed Mr Harris and his friends.
Justice Lyons said on Tuesday Webb had become progressively angry over a 40 minute period before the attack and furiously stormed off to confront Mr Harris.
She said Miller wielded a knife in a defensive manner and also kicked away a discarded blade during the incident.
"Mr Webb however, you did advance to Mr Harris, punching him vigorously to the stomach causing him to fall onto the roadway," Justice Lyons said.
"The footage shows sustained punches against someone who was alone and unarmed. It can only be described as cowardly and disproportionate."
Webb-Italia ran up to Mr Harris and pushed him against a tree and bin as he stabbed him at the start of the confrontation.
Webb-Italia and his brother further assaulted Mr Harris as he stood against temporary roadside fencing before collapsing on to the road and later died.
Justice Lyons said the "totally unnecessary" death was caused by drug-induced anger, disproportionate aggression about perceived insults and going to a punch-up with knives.
Webb-Italia was sentenced to life imprisonment for murder, with 1336 days in custody already served.
Justice Lyons sentenced Webb to nine years in prison for manslaughter, with 1335 days already served.
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