First off, well done to the BBC for making a documentary on the subject and making it a fair one too. I have the greatest sympathy for Sgt Blackman, who shot a wounded Taliban dead in Helmand province, Afghanistan in 2011 and is serving time for it.
He was in charge of a group of young marines who had been in heavy combat for day after day. Film from the scene showed that his young troops were angry at the wounded Taliban and were in no mood to treat him or call for medical help for him. One indeed offered to shoot him in the head.
Blackman was left with a wounded enemy to deal with and angry and restive troops under his command. What he was obliged to do was to call for medical aid. This would have involved five British vehicles coming up a route which was known to be lined with explosive devices, or calling in a helicopter to evacuate the wounded Taliban, into an area where there had been a firefight that morning and several over the previous few days. Either option would have put more British lives seriously at risk.
So he did what he did. Wrong, but I wonder what the legal eagles who put him in jail would have done in his shoes?