Let's look at the "Bonus March" of 1932. Over 20,000 WWI vets had marched on and encamped in Washington D.C. They were demanding early payment of service vouchers (due to be paid in 1945). The marchers had clashed with D.C. police, who had been overwhelmed. Three of the leaders of the marchers were known Communists trouble makers. When the House passed an early payment bill, but the Senate failed to pass the bill, the marchers became riotous. D.C. police could not handle the situation and there was no State militia or National Guard to call upon.
Hoover called on the Army to restore calm before a full-blown rebellion erupted. Douglas MacArthur ordered the Army to remove the illegal squatters. They did so without firing a shot! The Army fixed bayonets and the Cavalry drew their sabres, but there was no gunfire. The Army torched the illegal encampments and also used tear gas - a common enough police techniques. Although it was a public relations disaster for Hoover, the Army showed remarkable restraint.
In the case of the National Guard incidents at the Kent State and Jackson State in 1970, you are dealing with non-federal forces. In both cases, you had state National Guard units reacting in a defensive posture. I disagree with your statement "... they posed no threat to the Guard. At Kent State, they did pose a threat by their very numbers and actions, and in light of the events of the prior 24 hours.
At the Branch Davidian complex outside of Waco, TX, I think we witnessed a mass execution. I agree that the actions of the Clinton Administration to end the seige were an abuse of power and totally unjustified. However, the final "assault" was made by "police" forces and not military forces. Any military forces in the area played an auxilary role, at worst.