I would hope people that would choose to carry would be familiar enough with firearms to know better than to do that. Pistols were not going to do anything helpful. I might have felt better if I had been there if I was armed. Only because without knowing where the shooter or shooters were you might think you would encounter one trying to get away.
I don’t think anyone at the concert could have done anything if they had been armed, even with a rifle- unless they were a trained sniper already in a good position. The only ones that might have been able to do something if armed might have been people inside the hotel, near his room, if they had been able to get inside- likely they would have been shot just like the security guard but who really knows?
He was shot through mouth and chest. Had to be sniped, if swat did not shoot him.