That's not fair to the guy on the left. The guy being shot was an NVA spy, caught in enemy territory in civilian garb instead of a uniform. The Geneva Convention states that captured spies can be summarily executed, which is what is happening here. So this was perfectly legal, while we still don't know for sure about the Costco shooting (and I have my doubts).
I recall that the guy being shot was part of a “hit squad” of NVA during the Tet offensive, that had been captured from the scene of where the officer doing the shooting’s family had been slaughtered. It has been a while.