A senior detective investigating the murder of a man who confronted youths throwing litter said people are entitled to challenge bad behaviour.
Evren Anil died from a head injury
Evren Anil, 23, died eight days after being punched by a teenager who tossed a half-eaten chocolate bar into his sister's car in south-east London.
Detective Chief Inspector Cliff Lyons said: "People are entitled to challenge these yobs' behaviour."
Five people have been bailed over the attack in Crystal Palace on 5 August.
Mr Anil, 23, of Upper Norwood, south-east London, fell into a coma soon after hitting his head on the pavement during the attack, and died on Tuesday.
DCI Lyons, who is leading the murder investigation, said: "We have got a 23-year-old university graduate starting his way in life, with a new job and a close-knit family.
"He challenges their behaviour and this is what happens.
"When they produced the knife he tried to get back in the car. Your life is worth more than a chocolate bar."
Mr Anil was in the front passenger seat when the pair threw rubbish through an open window as the car waited at traffic lights on Central Hill.
Police said an elderly passer-by intervened when the youths produced a knife, but that person left the scene when the blade was put to his throat.
The passer-by then alerted the emergency services.
Mr Anil's sister Elif told BBC London: "What I and my family have been through, I can't describe it. It's very hard, very hard.
"I would not wish this upon anyone else. And this needs to be sorted out."
Both suspects, who were described as being black and in their late teens, ran off into the nearby Central Hill Estate.
Her brother had recently graduated with a first class degree in computer science from Kingston University.
DCI Lyons said there were people in the local community who knew the identity of the killers but had refused to come forward.
He said: "But we are not talking about a stolen bottle of whisky off the back of a lorry. We are talking about a young man's life."
Mr Anil's cousin, Mehmet Aray, said: "We have got youngsters running riot on our streets, carrying knives.
"But it is normal for us now. It is just another part of our lives."