It's weird that some think that shooting retreating (not surrendering) soldiers is somehow 'wrong'. As someone who studies WWII, this was commonplace then - you either walk towards the victor unarmed with your arms raised (a universal sign of 'I surrender') or you take your chances running.
Were I a warrior, I'd not think twice about shooting at someone retreating for the opportunity to kill me again. Surrender is different.
That said, none of this is relevant to the subject at hand, imo. Professional solders ≠ average civilians.
It was worse than that. Anyone trying to "surrender," after an active exchange of gunfire, who had no ammo on him was shot. The unwritten rule was that you could only surrender when you still had the wherewithal to kill someone. This is not widely known, but it was the rule. You didn't surrender unless you had a full clip, especially right after trying to kill someone. Harsh, but understandable under the circumstances. That said, the rules of war have no application here
at all. Brad is correct. The shooter's remarks are very damning. It was basically an execution. However, given the totality of the situation - his being body-slammed with broken bones, plus his assertion, if true, that they'd targeted him twice before, I'd be surprised if he's true-billed...