I had an officer ask me this exact question when I was a soldier. He assumed an order had been given to regular US Army troops to fire on United States citizens was given, and it wasn't an immediate action in self defense. I think my answer shocked him a bit. I told him, "Sir, If I ever receive any such order, the first citizen I kill will be the man that gave that order, I don't care what happens after that."
H5

Sounds like something my Dad would have done.
He was a Korean War vet, and had almost made sergent, when some snot-nosed lieutenant got in Dad's face, and got busted in the face by Dad. Dad was busted down to private, rose to corporal before the end of the war.
DUmmies should remember this line from a current country song:
There are only two men that ever died for me.
One died for my freedom, the other, for my soul...
God bless soldiers and Jesus.
But I'm sure this, like so much else in life, will simply pass
over through their pointed, little heads.