He didn't shoot them by accident, he shot them by negligence. There's a big difference. Legally, it's even worse than that. He was intending to shoot and kill the driver of the car, this intent is carried on to the inadvertent recipients of his gunfire, so in a just system he would be facing three counts of attempted murder. Self defence doesn't apply here, by the fact that he was a party of the protest that was impeding traffic, he (and his companions) created the conflict that he was supposedly defender against. There may well be other aspects of self defense law that he fails on too, but failure on any aspect is terminal to a self defense claim.