A youth worker was stabbed to death after being "taunted" by teenagers he confronted when he suspected they were dealing drugs outside his home, a court has heard.
Omid Saidy chased two males from outside his house near Parsons Green, south-west London, but was knifed in the neck, the Old Bailey heard.
The 20-year-old had gone after the suspected dealers with his friend Oluwafemi Omotosho, known as Femi, who was on a moped.
Shafiq Smith, 19, and a 17-year-old, who cannot be named because of his age, are charged with Mr Saidy's murder.
Smith is also charged with the attempted murder of Mr Omotosho.
Prosecutor Timothy Cray told the jury the police investigation suggested the victims as well as the defendants were carrying knives.
He said: "Should they have chased the defendants? Absolutely not. Dealing with drug dealers is the work of the police and the events show how quickly violence can spread if individuals try and take the law into their own hands.
"But did Omid need to die for the mistakes he made, and did Femi need to be stabbed in the chest when he was lying on the ground? Those are the sort of very specific questions you'll need to focus on as you approach the evidence and reach your decisions."
Mr Cray told the court: "Omid was angry that people were dealing drugs near (his) address."
He said the group had been asked to move on earlier in the day on October 16 but returned that evening with their faces masked or wearing hoods.
After an initial chase by Mr Saidy and his father Majid, who had come outside with the family dog to ask the group to go away, the defendants produced knives, the court heard.
Majid Saidy, giving evidence on the first day of the trial, said: "They were across the road taunting Omid with the knives."
He said he had tried to tell his son to "let it go" but heard him say: "I'm not having this, they can't be waving knives at me like that."
Majid Saidy said his son, who he described as a youth worker, ran after the people he believed had been outside the house.
He described seeing "a confrontation" and someone arrive on a moped but said he was not close enough to see what was happening.
Mr Cray asked him: "By the time you catch up you realise that Omid's been stabbed?"
He answered: "Yes. I could just see a pair of trainers on the floor and he is on the ground. I still don't know it's Omid."
CCTV footage played to the court showed Omid Saidy initially walking after the defendants as Mr Omotosho rode on his moped.
A fight involving Mr Saidy, Mr Omotosho and the defendants took place under a railway bridge - not covered by CCTV - on Parsons Green Lane near the Tube station, Mr Cray said.
Mr Saidy was stabbed in the neck and pronounced dead at 8.30pm, the jury was told.
Mr Omotosho was stabbed while lying on the ground after falling off his moped but survived, the court heard.
The defendants, sitting outside the dock, were 18 and 16 at the time of the killing last year, the jury was told.
Smith, of Laitwood Road in Wandsworth, and the 17-year-old, of Shildon in Durham, deny all the charges.