There has been a shooting in a gun store. This is a rare event. Yet, there's no real reason it would be.
Even gun stores usually have policies against loaded guns in the store.
It makes sense in a way. In a gun store people are going to be handling firearms in all sorts of ways-- checking to see how they fit in their hands, how the sight picture looks, how the slide or cylinder feels, and things like that. Some may even attempt to dry fire. So, for the guns that are going to be handled to be loaded would be a recipe for disaster.
But for holstered guns...
Yes, the employees of gun stores are often (if not usually) armed. This just means any bad guy knows who to shoot first without having to scope out the situation too much. It wouldn't surprise me to learn store employees were the ones initially killed. (Reports are still fragmentary as I write this, with no real word on what actually happened.)
As always, it's better if the bad guys either know everyone is armed, or don't know who is. Because, once again, a policy will never stop a bad guy intent on killing people from ignoring a sign on a door and just going through with what he wants to do.
I'm glad there were other armed people inside the store who prevented more deaths by engaging (keeping him busy) and killing the bad guy. Sometimes the bad guy is just going to do what he's going to do and the best anyone can hope for is to drop him before he makes things worse.
Seriously, if a bad guy is going to start shooting (or stabbing, etc.) near me, I'd rather be in a gun store than just about anywhere else.