Joshua Lindsay Elkerton-Sandy, 21, is set to walk free after a Brisbane Supreme Court jury took only a couple of hours to reach its verdict on Wednesday.
Security was beefed up at the courtroom following tensions between supporters of Elkerton-Sandy and Kane Alexanderson who was 18 when he died of a stab wound to the heart three years ago.
Tensions boiled over after the verdict with groups clashing outside the court building despite a heavy security presence.
At one stage Elkerton-Sandy supporters retreated back into the court building, sprinting to evade a rival group in pursuit.
A woman was restrained by a number of security in the court foyer.
Police soon arrived with a number of officers standing outside the court building.
The jury heard Elkerton-Sandy reached for a knife when a group including Mr Alexanderson barged into a Brisbane CBD high-rise apartment three years ago.
Members of the group had a "beef" with Elkerton-Sandy and planned an attack for days, defence barrister Stephen Kissick told the jury during the trial.
A nine-strong group including Mr Alexanderson had been drinking at a building nearby and left to confront Elkerton-Sandy who was at the apartment with friends to watch Australia Day fireworks.
The group snatched a security card for the apartment and seven members were able to barge their way in with Mr Alexanderson leading the charge, the court heard.
Some were armed with beer bottles.
After a confrontation that lasted barely a minute, the group fled with three members suffering stab wounds.
One was slashed in the face, another in the torso and arm while Mr Alexanderson was stabbed twice, including a 6cm deep wound that penetrated his heart.
He died six days later.
"There will be no ... contest that Joshua Elkerton-Sandy caused Kane Alexanderson's death," crown prosecutor David Nardone had told the jury.
"The question becomes did Joshua Elkerton-Sandy lawfully act in self-defence?"
Mr Kissick told the jury that Mr Alexanderson was part of an intoxicated mob who had committed a violent home invasion.
He said Elkerton-Sandy had reached for the first thing that he could see when the group entered and grabbed a knife.
"If seven people ran into your house unexpectedly, what would you do?" Mr Kissick asked the jury.
"This wasn't just a bunch of mates popping over ... it was a run-in gang bashing.
"He chose to stand up to this gang ... he was obviously entitled to defend himself."
Elkerton-Sandy looked skyward in relief when a not guilty verdict was read out.
The public gallery was quiet after Justice Sean Cooper had earlier warned they could not express their feelings about a verdict.
However, groups clashed outside the court building soon afterwards.
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