The Kent state shootings happened after hours of rock throwing and intense rioting by the "Peaceful" demonstrators there. Some of the Guardsmen there were getting hurt, and that was when they decided to open fire.
Yes, I know. In the weeks leading up to May 4th, the small campus was inundated with many outside Peace type groups including "The Weathermen" etc - I was in chemistry class when the shots were fired - it was terrible