The teenager gunned down an innocent bystander at point-blank range during an armed robbery on a corner shop in Hammersmith, west London, in February this year, the court was told.
The victim, Amaranath Bandaratilleke, 32, a legal clerk, known to his friends as Nath Banda, was visiting the family who ran the shop.
Dorian Lovell-Pank, for the prosecution, said Mr Banda was shot in the chest when left alone in the shop with the 16-year-old. 'The Crown's case is that this was a deliberate, cold- blooded shooting.'
The court was told the teenager later made a tearful confession to his sister, who told her parents. The father and son then went to Hammersmith police station so he could give himself up. The teenager claimed the sawn-off shotgun he was carrying went off by accident.
The 16-year-old, from Hammersmith, pleaded not guilty to murder. Sam Perman, 18, of Ravenscourt Gardens, Hammersmith, who is said to have taken part in the robbery, also denies murder.
The trial continues.