Lew Rockwell blog 
Dec 15, 2012
On his program the other evening, Piers Morgan asked if the 2nd Amendment defenders thought shopping malls could be made safe from shooters if everyone who came into the mall was armed.
This character misses the point: You wouldn’t have to have anyone in the mall armed. It would be sufficient — to dissuade a would-be shooter — that people were at liberty to carry a gun.
If others might be armed — and thus able to resist his attack — this possibility would enter into his calculation of the benefits and costs of his planned action. (I grow weary of listening to those who fail to take into account John Lott’s research on the topic.)