On the other hand, people I know who carry, myself included, are constantly aware of their environment - BECAUSE - they are carrying.
This ahole was completely armored but he walked up the aisle leaving 1/4 to 1/2 the theater audience at his back as he shot people heading for the exits.
An armed American might have been able to follow him and shoot him in the back - is most body armor 360 degrees?
If nothing else - from the rear, knock him down, stuff your pistol between gut and crotch (between armor), press hard and shoot - until he stops moving.
This will slow him down.
When you carry concealed, ignore the "no guns" signs at malls, stores, etc. EXCEPT as provided by state or federal law.
If the establishment is using frisk and metal detectors to prevent anyone from getting in with a weapon then fine - lock it in the vehicle, if not, do not disarm yourself for your family or others.
Trust me. ;-)
As a concealed carry holder one must be aware of their surroundings. It is always on my mind that my bullet is my responsibility until it comes to a stop. If it hits the target and goes through him and kills someone else you have a serious problem. Crowded areas can create heroes but it can also create felons.
Usually front and back. Side gaps are common for comfort but not universal. If the media got it right on where he bought his vest (in the link I followed), it was advertised as "tactical" but it was not actually bullet proof.