We need some real experts as mentioned like Ayoob, to go on and explain that this could have been done with any weapon, if a crazy person decides to kill people. He could have made incendiary bombs or pipe bombs or a hundred other ways to kill it he wanted to kill that bad.
You can't stop a determined person who doesn't mind dying from killing people, we first heard that in the Godfather, and the Kennedy assasination, no one can be protected 24/7. But if there would have been a dozen permit holders in there I am pretty darn sure one bullett would have found it's mark. Probablly would have saved a lot of dead people.
I know at that distance even with all the smoke, I could have put a laser on him. Maybe 1 or 2 seconds. Even if one got killed another shooter would have got him.
Gun free zones are only for law abiding people, to criminals they are" kill without retribution zones".