The question here is whether or not there were specific circumstances that justified deviating from that general law of war. If the guy was badly gut shot or clearly had a mortal wound that would result in his inevitable death shortly, I can see the argument for putting him out of his misery. Same if he presented a threat.
But I also can see why you've got to look at claims of such "mercy killings" pretty closely to avoid a de facto rule that all wounded enemy get shot. I mean was the guy really mortally wounded, or not? I dunno, because I don't know what the evidence was at trial.
I don't believe in kneejerk criticisms of actions taken in combat. Been there, done that. But I also don't believe that we should automatically justify everything a particular soldier may do unless we're sure we've got all the relevant facts.
Like everyone else here - I haven't seen the footage or the facts in full that went before the trial.
My tuppence worth is that this isn't a hysterical media led proscecution. This (as I undertand it) was a procecution brought by the military itself. The man is an officer. He is educated. He knows the line and so do his peers. Given the lack of political pressure behind this one - I have no reaseon to doubt the judgement that he must have crossed the line. I don't doubt he had anything but a fair trial prior to being found guilty. The system works. That why we fight for what we fight for.