Sure they had a choice. They had a choice whether to aim at the sky, the ground, or the protestors. And they had the same choice the protestors were exercising: whether or not to obey the authority ordering them to do stuff. But they weren't ordered to fire on the protestors. The guardsmen who did shoot at the sky or the ground, which was most of them, weren't prosecuted for disobeying orders. The more important choice was the first. That's the one for which they should have been charged with murder.
P.S. How rich and privileged are kids who go to Kent State in Ohio? It's not Yale. In 1970 it was pretty cheap. And regardless, why does this justify the harm they suffered? The guardsmen were right to shoot the college kids because they weren't as rich or privileged? That's a really horrifying take on the event.
That's another long tradition: portraying the victims of lethal violence as unsympathetic in some way as though this mitigates the harm they suffered.
I assume most people reading Hacker News aren't at the bottom of the economic ladder. Does this make them fair game?