So your scenario suggests that the 21 and older gun carrying law-abiding citizens that would be somehow present in this venue would be deterrents to this twerp who fucking rage quit and shot up the joint?
Would it be a deterrent? Probably not.
But there is no surefire "solution" to this problem. It's a societal issue.
But handguns, as it appears was used here, are simply never, ever, EVER gonna be banned in this country.
Ever.
Not in anyone's wildest dreams. It's not happening, so move on.
And in the search for the next idea, you know, there are plenty of examples of armed victims turning on their attackers and preventing deaths.