Honor, murder and "the law".

[AmericanPride]

Sounds a lot like the Malmedy Massacre on a smaller scale, which militarily was a "last resort" because the prisoners would have held up the SS advance on Stavelot (just down the Ambleve from Malmedy). Thus, a War Crime on the part of both the shooter and the theatre commander (IMO).

Changing the facts (adding more facts) might give a more nuanced response.

IIRC, a theater commander can order a legal reprisal in order to compel the enemy to conform to the laws of war.

EDIT: I believe this is relevant to the present conversation because of the obvious options which reveal themselves should the US decide to pursue said compellence. If that were decided upon, suddenly the shooter's action in this case is not murder, but (possibly) a legal reprisal. The act remains the same, but it's legality and (supposedly) its moral value change.