A teenager was hunted down and shot in the head for daring to confront his girlfriend's ex who had been bombarding her with "threatening" Facebook messages, a court heard.
Ben Purdy, 18, was allegedly killed because Michael Bagnall wanted to teach him and his friends "a lesson" after they decided to have it out with him.
Michael Bagnall is on trial for murder alongside family members Andrew and Reece Bayne and a 17-year-old youth, who cannot be named for legal reasons.
After a "skirmish" with the victim and two of his friends on the afternoon of November 23 last year, they jumped in a car and scoured the streets of Bermondsey, the Old Bailey heard.
They allegedly armed themselves with a self-loading pistol, a mace, a cosh or knife and an old fashioned police truncheon as they drove around in a Ford Focus looking for Ben, Callum Malone and Jack Walker.
When they caught up with them, Andrew Bayne shot Ben in the head while Reece Bayne chased Callum and cornered him, jurors were told.
Andrew Bayne then pointed the gun towards Callum telling him to open his mouth before Reece shouted out to him "We need to go - he's dead" and they made off, the court heard.
Ben had suffered "catastrophic injuries" and died in hospital the next day.
Opening the case, prosecutor Crispin Aylett QC told how the murder arose from a feud between Michael Bagnall and the victim who had started going out with the defendant's former girlfriend Hayley.
Despite the fact Michael Bagnall had found a new girlfriend following the split in July last year, it still "rankled" that Hayley had moved on and he sent her "threatening messages" by phone or Facebook.
Mr Aylett said: "After this had gone on for some time, Ben obviously had enough and so that Sunday, around lunchtime, with two friends to help - Callum and Jack - the three of them decided to go and confront Michael Bagnall and have it out with him.
"It was undoubtably a very stupid thing to have sought to do and cost Ben Purdy his life."
Shortly before the murder, there was allegedly a "skirmish" between Ben and his friends and three of the defendants - Michael Bagnall, Lee-Ann Bagnall, and Anthony Bayne close to the Bagnall family home in Bermondsey.
Although Ben was not carrying a weapon, his friends allegedly had a motorbike chain and a metal pole but still fled at the first sign of trouble, jurors were told.
Mr Aylett told the court that Michael Bagnall decided that Ben and his friends needed to be "taught a lesson".
Andrew Bayne, 37, Michael Bagnall, 22, Reece Bayne, 19, and the 17-year-old are charged with murder.
Michael Bagnall, Lee-Ann Bagnall, 19, and Anthony Bayne, 33, are charged with affray by threatening unlawful violence against Ben Purdy, Callum Malone and Jack Walker.
Michael Bagnall, Reece Bayne, and the youth are charged with conspiracy to commit assault occasioning actual bodily harm.
And Andrew Bayne is charged with possessing a self loading pistol with intent to cause fear of violence.
The defendants from Lewisham deny the charges against them and the trial continues.