From a moral standpoint, the right thing to do is for the armed bystander to intervene, to save lives. But from a legal standpoint, the basis of being armed is personal self defense. Every armed citizen is not a substitute, amateur, self-appointed policeman. As laudable as this person's actions were, if as a society we go down that road, it will ultimately lead to chaos.
And this armed bystander was lucky. In a similar situation, he could have made a mistake, an innocent person could have been shot, and then he could be subject to legal liability. If you're going to intervene, you damn well better be sure you are shooting the right person. The police have legal protections, for their actions, that ordinary citizens do not.