ROE are very important to scenarios like this one.... In my time, we dealt with some really bad ROE and sometimes they were unbelievably relaxed....
I would assume that this Marines ROE were similar to those of the majority in country.... If I remember correctly, this episode was caught on video tape.... After coming under attack from a village building, they entered and found dead/wounded inside... This guy Weemer assumed there was still a threat from this wounded insurgent and took appropriate action to eliminate the threat....
Putting down a possibly still aggressive wounded combatant is a part of the ROE... I would know...