Hot, tired and tense, the eight Royal Marines who made up Patrol 14 Hotel were heading back to base after a routine search of a compound in southern Afghanistan when the call came that an insurgent suspected of plaguing a British command post had been spotted in a cornfield.

The men crouched on the edge of the field as a British Apache attack helicopter – one of the few things Taliban fighters fear – flew in, and cheered as its cannons opened fire. They were surprised and frustrated when the message came back that the man may only have been wounded and the patrol was tasked with the hazardous job of walking out to where the heavily armed insurgent lay to check his condition.

The bravery and skill with which the marines tackled the situation is not in doubt. The insurgent's AK-47 assault rifle lay within his reach and there was a grenade in his pocket but they disarmed him professionally, clinically, without fuss.

What happened next, however, tarnishes the reputation of the British armed services and the Royal Marines.

Bleeding and moaning, the prisoner was dragged roughly to the edge of the field. He was abused and mocked before being dumped on a pile of chaff.

Once the Apache had clattered away, the man who led the patrol – an experienced sergeant – leant close in and shot the helpless man in the chest.

As the insurgent's body twitched horribly, the sergeant, who can be identified only as Marine A, told the man: "There you are, shuffle off this mortal coil, you cunt. It's nothing you wouldn't do to us." Moments later, A told colleagues: "Obviously this doesn't go anywhere fellas. I've just broken the Geneva convention."

Fiercely loyal, the patrol kept quiet – but unfortunately for them the whole episode had been captured vividly on a head camera worn by another of the men, Marine B. One year later the video was found by chance by military investigators and led to three marines, A, B and C, being charged with murder. A pulled the trigger but B and C were accused of encouraging and assisting him. A was found guilty on Friday, but the other two were acquitted.

The disturbing events of 15 September 2011 came towards the end of an arduous, brutal six-month tour, Operation Herrick 14. The marines, members of 42 Commando, were in Helmand to help build schools, hospitals, roads and help prepare Afghan forces for when they take full responsibility for security. But they also had to cope with a determined and motivated enemy.

Marine A and Marine C, the youngest of the trio accused of murder, were stationed at a command post called Omar. Their tour began relatively quietly, partly because the "$10 Taliban" (young men who fight for a few dollars a day) were at that time occupied gathering in the poppy harvest.

Marine B was busier from the off at a neighbouring CP (command post), Taalander, which came under fire from the Taliban almost every day. On one occasion he had to clean up after a friend suffered fatal head injuries in a grenade attack. "I picked my mate's brains off the floor," was his tough summary.

In May 2011 the marines suffered two devastating losses. A commander called Major Steven McCulley was hit by a roadside bomb, with the shrapnel fracturing six ribs and tearing through his right lung. He was evacuated, spent nearly three weeks in a coma, then two years in rehab, and has retired from the military.

Worse still, at the end of the month, another roadside bomb claimed the lives of two hugely well-liked and respected marines, Lieutenant Ollie Augustin, 23, and Marine Sam Alexander, 28.

Augustin was considered a brave and brilliant young officer with the brightest of futures; Alexander was something of a legend in the corps, having been awarded the Military Cross for charging down a group of Taliban insurgents with a pistol during an earlier tour. Others were injured in the roadside bomb, including one man who lost a leg. The marines were outraged to see it hung from a tree as a trophy.

As the harvest drew to an end and "fighting season" got under way, life became – to use Marine A's description – frantic. By September, partly because of the number of fatalities and injuries, the men under A's command at Omar only had one day of rest every two or three weeks.

Marines that had been keen to taste action were now not so willing to risk being shot out so close to the end of their tour and A began to go out on almost every patrol. He was feeling stressed and jaded. Marine C, who was "point man" on most patrols – in charge of the mine detection equipment -was tired and scared. "I was drained, very stressed and constantly in fear," he said during the court martial. After Omar suffered yet another casualty, Marine B was transferred from Taalander to Omar just three days before the disastrous patrol.

There was no laughing and joking when they went out that day. Marine B described the mood as focused, adding: "Everyone was in their own little world focusing on what they needed to do."

Marine A accepts he lost that focus and self-control when he shot the injured prisoner. However, he insisted he thought the insurgent was dead when he pulled the trigger and believed he was firing at a corpse.

The identity of the insurgent killed during that patrol is not known. But the marines believed he was one of the Taliban fighters who had been plaguing Taalander for months.

Another detail that emerged is how rare it was for the marines to come upon an insurgent at such close quarters. They usually only see them through the sights of their SA80 assault rifles and to actually lay hands on a combatant was by no means an everyday event.

The crucial shot fired, the insurgent was left where he lay. A shrine later appeared on the spot in his honour and a few weeks later the marines were packing up and heading out of Helmand.

Military chiefs celebrated the successes of Operation Herrick 14, which had included carrying out 41,000 patrols and seizing more than seven tonnes of home-made explosive. British troops killed 18 insurgents and captured another 19 who were on a target list.

The MoD said the marines helped to "strangle the insurgency" in Helmand and their link-up with their counterparts in the US marine corps was described as "historically significant". But British losses were heavy. Twenty-three service personnel lost their lives during Herrick 14 and dozens more suffered life-changing injuries.

On the day of an emotional homecoming in Devon Brigadier Ed Davis, commander of 3 Commando Brigade, said he felt humbled by the contribution made by all those who had served.

"Each person played their part in full and achieved something very special: hope for a safer world and confidence in a brighter future for the people of Helmand," he added.

During the year after the insurgent was shot, Marines A, B and C, got on with their careers. A helped train other marines in the use of heavy weaponry, his speciality. B and C, who maintained they had no idea A was going to shoot the insurgent, tried to forget what they had seen.

But there was to be no forgetting. In September 2012 one of the damning clips was found on another marine's laptop. Others were subsequently recovered from Marine B's helmet camera and a journal in which Marine C had written that he had asked for permission to shoot the captive was discovered.

The violence – and the coverup – were exposed.