I suspect that a person in that mall that had the right to carry a gun could have ended that shooting spree after one or two shots were fired.
That’s what happened in WA state last year, when an off duty out of jurisdiction cop ignored the “no guns” sign at a mall. He nailed a mass shooter before too many casualties. The shooter had a big sack of buckshot shells for his pump shotgun.
They all had the right. Maybe none had permission from their nanny, the gubmint, but they all had the right.