The wrong question.
The right one: “Under what circumstances should American soldiers fire on US citizens, and when should they refuse to do so?”
Let us look at a not totally unlikely scenario.
Next November Obama loses the election to Romney. Massive rioting and murder breaks out across the country. Thousands die and the rioting is ongoing and spreading.
Should National Guard and Army troops refuse to fire on the rioters because they are US citizens?
Not in Texas or other red states.