There is, of course, no way to know what the protesters at Kent State felt. However, it seems likely that they did believe that they were being patriotic.
Most people who oppose government policies feel that they are being patriotic. They tend to feel that they have the right idea about what America should be and that the government is the one that is doing wrong by the country. If you have this attitude, then protesting the government's actions is patriotic. If you love your country and you feel that it is being taken in the wrong direction, it is patriotic to oppose the government and try to bring the country back on the right track. Therefore, the protesters probably felt they were being patriotic.