Metal detectors and searches will hamper business too much for the malls to afford. And they would be the only way to really drastically lower the probability of an armed person in the mall. So basically, to stay in business, you gotta accept that there is the possibility of armed people in your mall, for good or ill. So in order to stop an incident like this, it needs to occur as soon after the initial crime occurence as possible. And logically, the best way to do that is by physically stopping the perp before he is able to shoot people. Now this can be done theoretically by tackling the guy, or using blunt weapons like purses. But we know the best way to stop a guy with a gun is another guy with a gun, preferably more. Since that particular solution is distasteful to the libs in charge of theses things, I predict they will go with the unarmed violence suggestion, just like they suggest at some colleges now. If one sees an armed individual shooting bystanders, tackle or hit them.
The rational thought of actually meeting the bad guys force with equal or greater force than your own will never be considered by the" experts" be in their minds, gun=bad. To them, it's as simple as that.