I can gurantee only one thing if every responsible citizen would be packing no idiot would enter a mall thinking he can enter and start shooting at people when he knows he will be shot back at. You ever seen gun store being rubbed? Nope because criminals know that everyone there is packing and you will get your ass shot.
How come you hear of mass shooting only in "gun free zones"?
Were that we could limit gun ownership to "responsible" citizens. Remember earlier this year in NYC, two police officers (who, at least in theory, should be better trained with their weapon than the average citizen) loosed 16 shots, hitting the target with seven.
The other nine rounds wounded nine civilians/bystanders.
2 dead, 9 wounded in Empire State Building shootings, police say - CNN.com
Imagine a world where people (reasonably) not as well trained as police officers are all armed.
And that's without entering into a discussion of today's tragedy.