A 15-year-old boy "paid with his life" when he was stabbed to death helping another teenager who had been robbed of his new trainers, a court heard.
Perry Jordan Brammer was stabbed 10 times on the Broadwater Farm Estate in Tottenham, north London, on 30 August and died five days later.
At Woolwich Crown Court Romario Lindo, 21, denied Perry's murder.
Prosecutor William Emlyn Jones said Perry "paid with his life" by "helping a friend of a friend".
The jury was told Perry had been spending the "sunny summer's day" with Jason Mombele, a friend of his and another unnamed teenager, who that morning had bought a pair of £90 trainers.
The court heard Mr Lindo stole the shoes from the teenager, who cannot be named for legal reasons, outside Bruce Grove Station.
In a bid to get the shoes back, Perry followed Mr Lindo to Willan Road, the court heard.
After Perry confronted Mr Lindo, he was stabbed repeatedly all over his body, the prosecution alleged.
'Don't kill him'
The attack was seen by a neighbour from a nearby building.
Mr Jones said: "As [the witness] watched - to his horror - he saw the defendant had a knife and was using it to stab Perry again and again.
"At one point, the witness shouted out, 'don't kill him'. In response, the defendant looked up at the witness, and then carried on stabbing Perry another three or four times."
The court was told the defendant then fled the scene on the bike Perry had been riding.
The 15-year-old had been stabbed 10 times in his face and neck and across his body, the jury was told.
Paramedics carried out emergency surgery before transferring him to hospital where he died on 4 September.
Mr Lindo, of no fixed address, has admitted robbery, but denies murder and possessing an offensive weapon.
The trial continues.