The Royal Marine convicted of murdering a wounded Taliban insurgent had been brutalised by the horrors of war in the "most dangerous square mile in Afghanistan" and had become paranoid that he was always the one being shot at, a court martial has been told.

Sergeant Alexander Blackman and the young marines under his control were effectively working behind enemy lines fighting insurgents that would have skinned them alive and beheaded them if they had captured them, his counsel told the court. He was tired and stressed, and also grieving following the death of his father just before his tour of Helmand began.

But Blackman's counsel, Anthony Berry QC, told the board which will sentence him that his overriding concern during the episode in which he shot dead the prisoner was safeguarding his own men and getting them back to base as quickly as possible.

Berry accepted that Blackman would be jailed for life and a minimum term of up to 30 years could be imposed – but he said it was also open to the board to jail him for just one day and asked for leniency. "The sentence should be so short that he can have a realistic anticipation of release in the not too distant future," said Berry. "That is because of the genuine exceptional nature, the unique circumstances of the case."

Blackman, 39, was convicted last month of shooting dead the prisoner who had been wounded in a helicopter strike. A video caught him telling the dying man: "There you are, shuffle off this mortal coil, you cunt. It's nothing you wouldn't do to us." During the court martial Blackman was identified only as Marine A but the high court ruled on Thursday that he should be named following a challenge from the media, including the Guardian.

Berry began by reading extracts from yearly assessments of Blackman that showed his capacity for leadership was spotted early on his 15-year career.

Just before he was arrested a report described him as a mature team player with "heaps of potential". Even as he waited for his court martial he was given glowing reports for his work training young marines in heavy weapons.

Berry went on to consider conditions in Helmand and what Blackman and his fellow marines were doing. As elite troops, they occupied frontline positions, effectively acting as a "lure" to Taliban fighters. He had 15 younger marines under his command and felt "immense" responsibility.

At the time of the murder in September 2011 the marines were frustrated. They faced long periods of "torpor" with temperatures of up to 50C making it hard to sleep – punctuated by what Berry called "intermittent periods of terrifying violence".

"They were subject to the relentless fear of what might happen to them," he said.

Blackman himself, as the men's commander, kept himself "apart from the lads" and was often on his own and isolated. He felt "mentally drained".

Berry quoted at length an article written by the film-maker and anthropologist Chris Terrill, who was embedded with the marines in the area after Blackman left.

In the piece, published in the Telegraph, Terrill described how the "god-forsaken" command post Blackman had been based at was effectively behind enemy lines in what was considered the most dangerous square mile in Afghanistan. There were so many mines in the area they had to play what they dubbed "Afghan roulette" whenever they went out on patrol.

Berry quoted from the article: "While the physical risk was undoubtedly enormous, the psychological threat was just as fearsome. The Taliban have never been averse to hanging the body parts of dead soldiers in the branches of trees – to taunt, to provoke, to goad. Often it was legs and almost always they were booby-trapped. There was also the knowledge that capture was a guarantee of torture – probably skinning followed by beheading."

He quoted Terrill's description of the "haunted, exhausted look in their [marines'] eyes after enemy contact." And how: "I have witnessed their night terrors following the elimination of their foes, and the grief and anger that grips them when comrades are lost or wounded."

Blackman's barrister continued to quote: "Soldiers are not automatons. They are flesh-and-blood human beings with frailties and vulnerabilities like all of us. They are ordinary people doing extraordinary things on our behalf; risking their lives in combat and having to make difficult and morally confusing judgments in the heat of battle. They don't always get it right because, sometimes, the stakes are just too high for any one man to cope with."

Berry went on to read extracts from a psychiatric report on Blackman. A consultant psychiatrist found Blackman private and plain-speaking. He was irritated when things happened beyond his control – such as poor equipment or the failure of the Apache helicopter on the day of the murder to kill the insurgent outright.

Blackman told the psychiatrist that his father had died just before his tour of Afghanistan began and he flew back in July – which would have been his father's birthday – to scatter his ashes. The psychiatrist also reported there was an element of paranoia in Blackman. He began to think it was always him that was shot at while on patrol.

Turning to the murder, Berry accepted that the marines "hated" the Taliban fighters. They hated the way they pretended to be friendly farmers but were secretly "armed to the teeth" ready to strike, and hated that they did not care if they killed children during their attacks.

But he said Blackman's "overriding concern" that day was to "make sure his troops got home safely". If he had called a helicopter to evacuate the insurgent, it would have taken 20 or 30 minutes to arrive, during which time they would have been exposed to danger. "The welfare of his own men led to the incident with which we are concerned," he said.

Berry ended by reading the testimony of Blackman's current commanding officer, Lieutenant Colonel Simon Chapman, who is based at the commando training centre in Lympstone in Devon.

Chapman said Blackman was a devoted family man and talented marine who, before what he called a "momentary lapse", had the potential to become a commissioned officer. He said Blackman was "extraordinarily proud" of being a marine but his future in the corps was now over. Chapman added: "He has my full support and will continue to do so."

Berry concluded by asking the board: "I urge you to be lenient."