An experienced marine sergeant faces a life sentence after a court martial found him guilty of the cold-blooded battlefield "execution" of a helpless, badly wounded Afghan prisoner – the first time in a generation that a member of the armed forces has been convicted of murder.

But despite the conviction the officer continues to be afforded anonymity and for the moment can only be identified as Marine A. He will have to be protected during his time in a civilian jail from possible attacks by fellow prisoners who it is feared might seek revenge for the killing.

Two more junior soldiers, Marines B and C, who were on the same patrol, were acquitted of murdering the captive insurgent and were ordered to return to their unit. The brutal incident had been captured on the head camera of Marine B, but the court has refused to release the film for fear of encouraging reprisals. Only the audio of the murder and a transcript of the video was made publicly available.

In the graphic footage, Marine A leans over and fires into the chest of the bloodied and moaning insurgent with a pistol. He then tells him: "There you are, shuffle off this mortal coil, you cunt. It's nothing you wouldn't do to us." A few moments later Marine A is picked up telling colleagues: "Obviously this doesn't go anywhere fellas. I've just broken the Geneva convention."

Colonel Richard Kemp, a former commander of British troops in Helmand, said the incident would make life more dangerous for troops. "This murder is a grotesque violation of British forces' longstanding reputation for morality and humanity even in the most desperate battlefield conditions," he said. "Execution of prisoners of war is a crime we associate with Nazi stormtroopers, not Royal Marines."

On the film seen only by the court, the insurgent was dragged roughly from a field after he had been injured in an air strike from a British helicopter. Members of the eight-man patrol are heard abusing and laughing at him. Marine C twice suggests shooting him before the man found guilty of murder does so.

General Sir Mike Jackson, a former head of the army, said: "Whoever we are, we are subject to the law. The due process of law has taken place." But he said it was worth bearing in mind that of the 100,000 troops who had served abroad in the last decade, Marine A was the only one who had committed murder. "A sense of perspective is called for," he added.

As well as putting the marines under the microscope, the court martial at Bulford in Wiltshire has focused attention on the principle of open justice. The three marines were granted anonymity by the court during the trial because of fears their lives be in danger if their identities were known. At all times they were hidden from the press and public by screens, an arrangement normally only afforded to special forces.

Judge advocate general Jeff Blackett, who heard the case, eventually agreed with representations from the media, including the Guardian and the prosecution, that they should be named, but the three marines are continuing to fight for their anonymity and the issue will be considered by the court martial appeal court. The Ministry of Defence has made it clear that it believes they should be identified.

Marine A had claimed he thought the man was already dead and was firing into a corpse in frustration. Marines B and C insisted they were shocked when Marine A opened fire and had not helped and encouraged him, as the prosecution argued. Marine A is the first British serviceman or woman to be convicted of murder since paratrooper Lee Clegg was found guilty of shooting dead two teenage joyriders in Belfast 23 years ago. Clegg was cleared on appeal.

In September 2006, an army corporal, Donald Payne, admitted a war crime connected to the notorious incident in which 26-year-old Iraqi hotel receptionist Baha Mousa died while in custody. But Payne was convicted of inhumane treatment rather than homicide and jailed for a year.

Defence sources said they knew of nothing similar to Marine A's crime in modern times, certainly since the second world war. The judge advocate general described it as "novel and new".

The marines stood and wore their green berets to hear the verdicts. Their reaction could not be seen because they remained behind their screen. Family members in the body of the court sobbed and held hands as the verdicts were returned by a board – the equivalent of a jury – made up of marine and navy officers.

Blackett ordered a psychiatric report to be prepared on Marine A before he sets the minimum term he will face. Meanwhile he will be held in custody. The judge told Marines B and C: "Return to your normal place of duties."

The trial gave a vivid insight into the pressure personnel were under in Afghanistan. They suffered devastating losses during their six-month stint in Helmand in 2011. In one particularly gruesome incident, insurgents hung a British marine's leg in a tree as a trophy after he lost the limb in a bomb blast.

Marines B and C were on their first overseas tour. The Service Prosecuting Authority, which brought the case, said: "This was always going to be a complex trial which polarised opinions as to actions on the battlefield."

The Royal Marines, an elite force that celebrates its 350th anniversary next year, apologised. Brigadier Bill Dunham, deputy commandant general, said: "What we have heard is not consistent with the ethos, values and standards of the Royal Marines. It was a truly shocking and appalling aberration. It should not have happened. It should never happen again."