There is no evidence yet released that anyone was slaughtered.
In fact, they will not even have bodies, if you believe this morning's news. The family will not permit them to be exhumed.
Let's determine: (1) That these GI's actually killed someone, and (2) That there was no battlefield explanation that mitigates the killing.
The court martial will determine that. If both those elements are present, then and only then should the USMC come down upon them like the wrath of Almighty God.