Apologies Luckygirl if that is not what you meant, but you kept referring to him as a 'young man' and about young men from chaotic backgrounds, lacking intellectual skills, joining the Army.
He is not that young, he is 41 and has presumably risen through the ranks to Sergeant and, as I said before, in a senior role of responsibility; Sergeants typically are second in command of a troop or platoon of up to 35 soldiers, with the important responsibility for advising and assisting junior officers.
He would have been in overall charge as no senior officers visited the outpost (!), his junior lieutenant (whom *he had mentored*) was killed. He was in charge day and night, fearing that they would be killed by stealthy insurgents during the night.
He was diagnosed as mentally ill after his trial as having an adjustment disorder caused by extreme stress, by a Professor of Psychiatry. "In Sgt Blackman's case he did not have a mental assessment before his trial, something that is routine in civilian murder cases."