Jimmy was killed a day after his 16th birthday
Schoolboy Jimmy Mizen was fatally injured when a fight broke out in a bakery, an eyewitness told a court.
Philip Lloyd told the Old Bailey that an "extremely aggressive" Jake Fahri, 19, punched Jimmy's older brother Harry in the bakery in Lee, south London.
In retaliation 16-year-old Jimmy and his brother charged at Mr Fahri and hit him on the head, the court heard.
Jimmy bled to death when he was hit in the neck with a glass dish. Mr Fahri denies the murder on 10 May last year.
Giving evidence Mr Lloyd said he left the bakery to call the police as the confrontation escalated.
Mr Lloyd said Mr Fahri challenged the brothers to a fight.
"The accused was verbally and violently abusing Jimmy Mizen," he said.
The argument turned into a fight when Mr Fahri hit the brothers with a plastic cold drinks bottle, but they retaliated when Harry Mizen, 19, was punched on the cheek.
"They charged at him and ran into him and careered him across the shop. They hit him on the head a couple of times," he said.
All three crashed into a cake display and the brothers shoved the accused out of the shop, but Mr Fahri smashed the bakery's glass door with an advertising sign and came back in.
"He was jabbing the sign towards them like a lion tamer. I saw Jimmy Mizen grab the end of the advertising sign and get hold of it," Mr Lloyd said.
When he left the shop to call police he met another of the schoolboy's brothers, 27-year-old Tommy Mizen, and as he explained what was happening the accused came out of the store.
Mr Lloyd said: "He was walking with a swagger.
"Tommy chased after him. He could not catch him."
Tara Gaynor, a passer-by, told the jury: "When he ran past me he had a big grin on his face.
"That is something I distinctly remember... He was smiling. It was disturbing."
Earlier in the trial Tommy Mizen described how he followed a "trail of blood" which led him to his injured brother who collapsed in his arms soon after.
The trial continues.