Let's face it, the guy was a combatant in a hot zone.....if he had been walking down the street with his arms in the air, the press would have shown it.
These are Marines, not police officers: you kill until your zone is safe.
even if he had been walking with his hands up - shooting him would be OK with me. the opening sequence in saving private ryan was pretty frank in this regard - after getting off the beaches, US forces shot many of the germans as they "surrendered".