This is a good article on the subject: “The real reason for shooting back.”  It deals exclusively with the mass shooting arena.  Bottom line, in gun free zones, the killer keeps killing until someone shoots back.  The longer it takes for an armed response to occur, the higher the number of casualties.  Personally, I have try to remind myself that whenever I see one of those signs banning CCW, I call the place a target rich environment.  Walk into a store with that mindset and see if the racks of goods take on a different appearance.  They are no longer the tempting deals, colors, or sale of the day.  They are the block between yourself and a bullet.  With no defense, that is all you have.