Having been there, three wars, 3 1/2 years in combat missions (Infantry Ranger), here is my take:
Violation of the Laws of War: Yes
Justified : Yes
Morally correct : No
Would I have done it: Probably not (wasn't there can't be 100%)
Situationally correct: No. The intel that may have been gained through interrogation may have saved the lives of many more American soldiers than the sniper ever could have taken. These men in WWII were Brigade or Division assets with enormous intell that could have tremendous impact for our side.
The laws of war state that when a soldier is no longer a combatant they are to be captured; that is the law. The Laws of War state that a surrendering soldier is no longer a combatant; that is the law.
Reality is, surrendering soldiers were used as means by the surrendering side to kill our soldiers. (that doesn't appear to be the case here given the stated circumstances). Fear often overrides our moral rational.
Just my take, worth what you paid for it.