Guns aren't allowed in bars for good reason. 

 I feel for the families and officers injured. I am glad they killed this idiot.

I don't know if someone with a concealed LEGAL gun could have stopped the guy. I am only guessing here, but a majority of gun carriers have no real training or experience of 300+ panicked people stampeding while rounds are flying. 

Even trained police and combat hardened soldiers would have had difficulty in this situation. 

At least taxpayers won't be on the hook for that bastard. 

How do you stop this type of event?  You can't. Nuts that want to kill will kill.