Wow! Some really intricate legal and moral issues involved here.
First off, it appears (to me, anyway) that the shooter was within his legal rights to draw down on the bad guys because at least one of them was displaying a weapon (a shotgun). It doesn't matter that the shooter was not the person being robbed. Deadly force can be used to defend against deadly force against others.
Now, having said that, last time I looked, vigilantism is against the law - which puts two different aspects of society on a collision course with each other in a society that allows concealed carry of weapons. Suddnely, in such a society, there are a whole lot of potential vigilantes walking around all over the place, aren't there? And what happens when they act as such? That would seem to be the question in this case.
I'm not sure I think that our society would be better off with a bunch of armed, potential vigilantes out there. Which is worse - that a store gets robbed or that people die? Now, we are getting into a philosophical position where reasonable minds can certainly differ.
I don't think that the "prohibition against guns" in the store in this particular case is going to mean anything. Under the facts of this case, I say shooter wins. But, that is not to say that I endorse such activity.