A teenager who stabbed an 18-year-old “peacemaker” to death following a dispute outside a McDonald’s restaurant 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 Ben Moncrieff, who was killed after intervening in a disagreement between teenagers in Bath city centre.

Bristol Crown Court heard Mr Moncrieff had stepped into a row between the defendant and another boy in the early hours of May 6 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 on May 6.

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”.

“He was out and about with at the time with older people who had possibly a negative effect upon him,” he said.

“It is very difficult during the course of the trial for expressions of remorse to take place but those who have spoken to him to say there is some level of awareness.”

Imposing a life sentence, Judge Peter Blair KC, the Recorder of Bristol, said the youth would serve a minimum term of 15 years’ custody.

“There have been some lovely things said about Ben Moncrieff in the impact statements, which I have read before coming into court and the one read by his mother in court.

“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,” the judge said.

“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.”

Detective Inspector Mark Newbury, who led the investigation, said: “At 18 years old, Ben had his whole life ahead of him, but it was snatched away.

“Tragically, Ben happened to be in the wrong place at the wrong time. He didn’t know the defendant and was entirely blameless.

“He was bravely standing up for a friend when the defendant made a conscious decision to pull out a knife and attack Ben knowing it would gravely injure him at the very least.

“We believe the defendant sought to attempt to glorify knife crime by posing in social media videos in the days leading up to Ben’s death and also showing off he was carrying one in the minutes before he killed Ben.”