According to
Wikipedia:
The Kent State incident forced the National Guard to re-examine its methods of crowd control. The only equipment the Guardsmen had to disperse demonstrators that day were M1 Garand rifles loaded with .30-06 FMJ ammunition, 12 Ga. pump shotguns, and bayonets, and CS gas grenades. In the years that followed, the U.S. Army began developing less lethal means of dispersing demonstrators (such as rubber bullets), and changed its crowd control and riot tactics to attempt to avoid casualties amongst the demonstrators. Many of the crowd-control changes brought on by the Kent State events are used today by police and military forces in the United States when facing similar situations, such as the 1992 Los Angeles Riots and civil disorder during the aftermath of Hurricane Katrina in 2005.
The guard were just following standard procedure of the time.