I would allow anybody under arms to defend themselves if they felt threatened while in the performance of their duties.
There is credible evidence of a gunshot before the fatal volley at Kent State. Personally, if someone was throwing bricks, rocks and molatov cocktails at me and I heard a gunshot, I would probably react in a most agressive manner.
Just looked it up. Seems there was someone there with a pistol. http://www.cleveland.com/science/index.ssf/2010/10/analysis_of_kent_state_audio_t.html