I read in one of the posts that the police dont train for an active shooter situation. They do. It may not be called "ACTIVE SHOOTER" but police train for, and deal with "man with a gun" calls all the time. The difference is some departments when confronted with an active shooter would wait for SWAT or some special unit leaving the shooter to have his way with the victims.
My department had no such policy. If we responded to a man with a gun call, we would investigate and attempt to apprehend or shoot the shooter. At the same time calling in the troops. Coordinating response, covering exits, and using every approach. Police 101. 99% of the time a 911 operator would keep a caller on the line feeding us information such as location and description of the shooter, victims needing aid, descriptions of others involved. Shooters active or otherwise would be dealt with the officers assigned to the call, or the first back up unit on the scene. If the situation becomes a hostage situation then the Emergency Services (swat) would take over along with a negotiator. If the situation became prolonged or there was a need for a detailed search, the first responding officers would be relieved by ESS officers if possible.
As far as an armed non-sworn civilian, or an off duty officer... If I was either... I'd probably take the shot.
For those who would take off... I'd advise looking for emergency exit signage. I'd advise against going to the parking lot or main mall entrance. It may be an organized operation by trained terrorists. It's a tactic they use to draw in the emergency responders en masse, along with spectators, and victims recieving aid. Then detonate a powerfull bomb. That being the main event, and the shooting just a way to draw in the actual targets