Hard to tell what happened there.
Could be they were battlefield casualties who were not recovered by their own forces immediately, too. Painting them as "executed" would be spin.
It is also SOP in the heat of battle, where the rear lines are not accessible and/or personnel cannot be spared to guard or escort to take captives behind a tree, or rock, and "incapacitate" them. Otherwise they'll be a burden, or bite you on the butt if you turn them loose.
Done by everyone.
Arf