A teenager who stabbed a "peacemaker" to death following a dispute outside a McDonald's restaurant in Bath has been detained for life.
The 16-year-old, who cannot be named for legal reasons, was told he would serve at least 15 years for murdering 18-year-old Ben Moncrieff.
Mr Moncrieff's "unwarranted and untimely" death was described by the judge as "devastating to the community" after he was killed when he had intervened in a disagreement between teenagers.
Bristol Crown Court heard Mr Moncrieff had stepped into a row between the defendant and another boy in the early hours of 6 May and was invited to "come around the corner" where he was stabbed.
He had been enjoying a night out with his girlfriend and friends when he suffered the fatal chest wound at about 3.30am.
He died of his injuries at the scene, despite the efforts of people in the area and medics to save his life.
The defendant, who was under the influence of nitrous oxide, had brought the knife to the scene in his waistband. He then fled the scene and dumped the knife.
Mark Cotter KC, prosecuting, told the court: "The crown's position is that the defendant entered into an argument with a friend of Mr Moncrieff and Mr Moncrieff entered as a peacemaker."
The defendant, who was 15 at the time of the attack, was convicted of murder following a trial.
He had previously pleaded guilty to possession of a bladed article in a public place.
Jeremy Wainwright KC, defending, said the teenager had been diagnosed with ADHD and had acted "impulsively" and had made "inappropriate decisions".
Imposing a life sentence, Judge Peter Blair KC, the Recorder of Bristol, said: "The effect of Ben's wholly unwarranted and untimely death has been completely devastating to the community of people he was a part of in Bath and his memory will live on.
"As a consequence, no one is going to remember the child in the dock whose senseless act has robbed them of a life."
The judge added: "Ben Moncrieff intervened to calm it down but you wanted to show off and in an act of stupid childish bravado you didn't want to forget.
"I accept your contact with Ben Moncrieff was minimal, but you took a knife to the scene and it was your actions that led to a penetrating injury to his heart."