If someone starts shooting randomly in a mall eventually bullets will hit those pesky things like children and women - no-one wants to be around flying bullets let it be house, car or restaurant.
Most of the times having a gun preemptively stops a situation from escalating, but when the sh!t hits the fan - I want to be one slinging, not hiding by the fan.
Big difference between a mall and a passenger jet. I don't have any problems with armed citizens in a mall. But not in a passenger aircraft.