Two teenagers have admitted killing a man in a row over litter.
Evren Anil died from a head injury eight days after the incident
Evren Anil was threatened with a knife and punched when confronting one of the youths who threw a half-eaten chocolate bar through his sister's car window.
Mr Anil, 23, hit his head on the pavement during the row in Crystal Palace, south-east London, last August, and died eight days later.
The youths, aged 16 and 17, who cannot be named, pleaded guilty to manslaughter at the Old Bailey.
They were remanded in custody and will be sentenced on 4 April.
In a victim impact statement Mr Anil's sister, Elif, called the teenagers "two heartless cowards" who carried out a "vicious and callous attack".
Mr Anil, of Upper Norwood, south-east London, suffered head injuries and fell into a coma as a result of the attack on 5 August, 2007.
He was with his 26-year-old sister, who was driving the car, when teenagers threw rubbish through an open window at a set of traffic lights.
Miss Anil said the incident had "shattered" her family, resulting in her parents separating.
She added that her brother's death had left her suffering from depression, and had caused her other brothers to require psychiatric treatment.
Miss Anil said: "I have been unable to sleep at night as the recollections give me nightmares."
The 17-year-old admitted throwing a chocolate bar wrapper through the window of Mr Anil's car, the court heard.
Jo Korner QC, prosecuting, said: "Having done so the defendant was confronted by the deceased and a verbal dispute ensued.
"There was some physical contact between the two involving the deceased grabbing the defendant's T-shirt. This resulted in the defendant punching the deceased."
She said the 17-year-old also admitted taking out a knife to frighten Mr Anil but claimed he did not intend to use it, then passed it to the other teenager.
Miss Korner said the 16-year-old's version of events, that "at no stage did he threaten Mr Anil with a knife" and only used the weapon to threaten another man, was not accepted.
The 16-year-old from Thornton Heath, south-east London, pleaded guilty to manslaughter and knife possession as his trial was due to begin at the Old Bailey on Monday.
The 17-year-old from Tottenham, north London, admitted the two charges on Tuesday.
Judge Ann Goddard said: "It seems to me custody is the appropriate outcome."
Following the incident Det Ch Insp Cliff Lyons said: "He (Mr Anil) challenges their behaviour and this is what happens.
"People are entitled to challenge these yobs' behaviour.
"When they produced the knife he tried to get back in the car. Your life is worth more than a chocolate bar."
Mr Anil, who was of Turkish origin, was in the first week of his job after graduating with a first class degree in computer science from Kingston University.