A 15-year-old boy shouted "you fat ugly p****" at a judge as he was found guilty of murder.
The boy, who cannot be named for legal reasons, beat 55-year-old Christopher Molloy to death in a gang attack in the early hours.
The boy stood trial for murder alongside Jay Byrne and Joseph Byrne, who had previously admitted Molloy's manslaughter, at Liverpool Crown Court.
Byrne and Byrne were cleared of murder but a jury found the boy guilty via a majority verdict following three hours and 41 minutes of deliberations.
The verdict prompted the boy to shout: "you fat, ugly p****" at the judge then slam a door as he stormed out of the dock, while Jay Byrne appeared to try to calm his co-defendant down, saying: "Chill bro."
During the trial jurors heard that there was an initial altercation near to a Burger King in Bootle, Merseyside, in the early hours of 12 July last year after Molloy was accused of "touching" the teenage defendant's girlfriend.
Jay Byrne, from Southport, was said to have either punched or pushed the stranger, who fled to the McDonald's opposite and sought help from staff.
Molloy stayed in the doorway of the fast food branch for several minutes before walking away, at which point he was chased by 25-year-old Joseph Byrne, of Litherland, and the 15-year-old, who was 14 at the time.
A second confrontation was captured on CCTV footage, showing the younger Byrne "landing an extremely forceful punch" that knocked Molloy to the floor, rendering him unconscious as he hit hit head on the pavement.
The boy was then seen repeatedly slapping him in the face, a total of eight times, as he lay helpless on the floor, before both attackers walked away, fist bumping as they went.
The boy returned shortly afterwards alongside 26-year-old Jay Byrne, who kicked the still-unconscious victim to the head. The youth then delivered a second kick to the head, described as a "particularly ferocious kick, like kicking a football hard".
After the attack, a taxi driver flagged down a police car to report a man lying unconscious in the road. Another member of the public had alerted the attentions of a passing ambulance, and the court heard that it became "immediately obvious that Mr Molloy was extremely unwell".
He was rushed to hospital where he underwent emergency surgery after suffering internal bleeding and fractures.
But later his life support was ultimately withdrawn and Molloy died on 19 July, a week after the assault.
Joseph Byrne told the court that he had given Molloy "a little slap to teach him a lesson" after he was told that he had been "grabbing the a***" of the teen's girlfriend.
The unnamed boy appeared to become angry with his co-defendant while under cross-examination - at one point turning to him and saying "he done it and he knows he did" - before refusing to answer any further questions from the prosecution.
Jay Byrne did not give evidence to the trial, but the boy said from the witness box that he had been imitating a scene from 1995 comedy film Friday when the character played by Chris Tucker shouts "you got knocked the f*** out". He also alleged that Joseph Byrne had asked "do you want to see a knockout punch?" before rendering Molloy unconscious.
Asked about the moment he kicked him while on the floor, the teen said: "I didn't think. I was just copying what Jay did."
All three males are due to be sentenced back at Liverpool Crown Court on 16 March.