IIRC, the last study I read on that subject showed that less than 30% of soldiers can be counted on to reliably fire their weapons back at an enemy. As above, that is indeed "fire back in retaliation". Additionally, while the idea that murder cannot be driven by insanity is ridiculous, I don't think insanity can be attributed to the whole of the deployed National Guardsmen if they open fire on civilian refugees. Haditha was a revenge killing.
However, the fire-ratio data was gathered almost 100% via interview with soldiers. While this does not make it unreliable in and of itself, it does lack physical evidence. While it is accepted as reliable in most circles (even influencing military training, who employed effective countermeasures against the "problem"), it's unfortunately not truly conclusive.
If our National Guardsmen actually open mass fire on those civilians as the worst-case scenario suggests, there will be absolutely be a global reckoning. This wouldn't be the same as the murder of 25 civilians during a time of war, or even the genocides in Africa. We would be talking about a modern-day Kristallnacht-level event.
Also:
It is expected the Pentagon and US Northern Command will amend the current document detailing the rules governing the "use of force" on the border mission.
Uh, how, please? In what way. Specifics.