Protesters were throwing rocks at the Guardsmen, and in the previous three days there had been vandalism, looting, and they had set the ROTC building on fire. From wikipedia.

From the Kent State website:

Friday evening in downtown Kent began peacefully with the usual socializing in the bars, but events quickly escalated into a violent confrontation between protestors and local police. The exact causes of the disturbance are still the subject of debate, but bonfires were built in the streets of downtown Kent, cars were stopped, police cars were hit with bottles, and some store windows were broken. The entire Kent police force was called to duty as well as officers from the county and surrounding communities. Kent Mayor Leroy Satrom declared a state of emergency, called Governor James Rhodes' office to seek assistance, and ordered all of the bars closed. The decision to close the bars early increased the size of the angry crowd. Police eventually succeeded in using tear gas to disperse the crowd from downtown, forcing them to move several blocks back to the campus.

Members of the Ohio National Guard were already on duty in Northeast Ohio, and thus they were able to be mobilized quickly to move to Kent. As the Guard arrived in Kent at about 10 p.m., they encountered a tumultuous scene. The wooden ROTC building adjacent to the Commons was ablaze and would eventually burn to the ground that evening, with well over 1000 demonstrators surrounding the building. Controversy continues to exist regarding who was responsible for setting fire to the ROTC building, but radical protestors were assumed to be responsible because of their actions in interfering with the efforts of firemen to extinguish the fire as well as cheering the burning of the building. Confrontations between Guardsmen and demonstrators continued into the night, with tear gas filling the campus and numerous arrests being made.

While the rally that was taking place at the time of the shootings was peaceful, I don't think it's any stretch to say that the protesters had displayed a considerable amount of violence and disregard for public safety and the law. The shootings were tragic and totally unnecessary, but the whole reason that the National Guard was there was because things were getting out of hand.