Actually, I for one, and no doubt most people, would not want any soldiers shot after capture.Again, it is a convention because we would not want our soldiers to be shot after capture.
Considering the lengths to which some go to avoid capture, possibly not quite the soft option you imagine.War crimes, eh. Severe penalties by UN!
Not just the U.N.Severe penalties by UN!
"Through international treaties the UK has a responsibility to investigate and prosecute those who have committed core international crimes."
Last but far from least I will mention the mental health of the perpetrator. Some years ago I saw an old man in a pub start crying. I asked if I could be of help to him. He eventually recounted his murdering a German prisoner in 1940. All those years later he was finding the emotional pain an awful burdon. I spent some time with him and found it a powerful experience and unlike your 'armchair philosophizing' quite unforgettable.