I certainly am not condemning the Marines in question. It wouldn't be the first time nor will it be the last that civilians are caught in the crossfire of an ambush, and there is certainly no evidence I am aware of that the Marines were even responsible for shooting any civilians at all --- as far as we know, which is not much, the Taliban shot the civilians in the ambush, not the Marines. The only way to know for sure is to autopsy the civs and determine whether they were killed by 5.56mm/7.62x51mm or 7.62x39mm --- that is, we will probably never know.

But to assume automatically that the Taliban were even capable of spiriting away the weaponry to achieve an IO victory, frankly, imputes a lot more competence to the enemy than I have seen evidence of.