I saw a former soldier on the news before saying, leaving aside the moral issues of executing a POW, it doesn't make military sense. He should have been debriefed to see if he had any crucial intelligence and if the enemy thinks they may be executed on sight while injured, they're more likely to fight to the death rather than surrendering. Surprised veterans are applauding this on that basis alone.
on the very specifics of this case, there was no prospect of the enemy combattant being debriefed/interogated because he'd been cut in half by a shell from an Apache helicopter - the bloke was in line to have only one more conversation, and that would be before the pearly gates.
the reaction to the case is complicated and multi-faceted, and in truth only a small proportion of the disquiet is based in the 'soljas is earoes in ay' crap that you see in the media - one of the concerns is that the first Defence legal team appear to have decided he was guilty of Murder before they met him, and unlike in pretty much any other murder trial they made no attempt to seek medical advice of what Blackman's mental health state was at the time, and therefore never even asked the judge in the first trial to consider whether diminished responsibility was appropriate - it was either Murder or innocent, and he wasn't innocent.
the second concern - which is a wider issue than this trial- is that the MOD, without whom Blackman would not have been in Afghanistan and armed with a rifle and standing over a man who had tried to kill him, threw him to the wolves. the MOD, and wider government, takes the view that when all is going well we're all a big team, but when shit goes wrong you're very much on your own. this, in very practical terms, means that anyone facing any kind of enquiry into their actions has to pay their own legal bills - bills of £300,000 for even short and inconclusive interactions with the utterly discredited IHAT are not uncommon, and for most soldiers they are bills that see them lose their homes. in this case those bills will be paid by the media and supporters because of the high profile nature of the process, but for the overwhelming majority of soldiers who face an enquiry there is no one to pay those bills but them.
the third problem - as alluded to by others - is that when shit goes wrong the prople who create the circumstances in which shit goes wrong won't be seen for dust. if Blackman is judged to be of diminished responsibility because of the outrageous workload he had faced in the months building up to this incident, where is the PM who set stringent troop limits and wouldn't allow reinforcements to be sent which would have reduced that workload?
the forth, related to the second, is anger at the culture of the MOD. some here -
Sasaferrato and perhaps
ViolentPanda - may recall an incident that occured at the end of the Falklands War in 1982: an Argentine POW was involved in moving Argentine Air Force bombs filled with Naplam from next to one of the field hospitals/POW camps, and one of the bombs detonated. the Argentine POW was completely immoliated in the detonation but was obviously still alive. one of the guards shot him twice in the head and he died instantly.
the injuries that POW sustained meant that he could have found himself in the burns unit of the finest hospital in the world attended to by 50 of the finest burns surgeons in the world and he still would not have survived the day - 400 miles out in the south atlantic and in a wet field he stood no chance. the senior Argentine officer at the POW camp thanked the guard for his merciful and swift action in writing, and the Argentine Govenment wrote to the British Government to say the same thing. the MOD on the other hand hounded the soldier for the rest of his career with investigation after investigation until he eventually left - at which point the MOD handed the matter to the civilian police who did more of the same.
Blackman is not some maligned hero, but he is a totemic example of how soldiers are treated by the MOD and the wider justice system when things go badly wrong - and the support for Blackman is perhaps better viewed as being anger at not just the MOD's 'throw them to the wolves' attitude, but also the MOD's encouragement for those wolves.