Be situationally aware at all times, but especially in a crowded area - know where the cover and concealment are located (there is a big difference); know that you can not protect everyone there, but your responsibility is to your family and yourself first (if you can get them out, do so); if you can see the shooter and can make a clean kill shot, do so, but do not put yourself at risk in the process by having to move from cover; keep in mind that there may be more than one shooter; when police arrive, you will hear them, and you will hear people yelling to them where the shooter(s) are; you don't want to be standing there with your weapon in hand when they arrive on scene - their adrenaline may get you killed, even if you are in a low ready position toward the shooter, because they have not yet been able to determine just who the shooter might be at that point...
Ultimately, if you are face-to-face with the shooter, it is "Gunfight at the OK Corral" time, so shoot and move at the same time. Make the shots count. Fast misses don't do a lot to a bad guy.
So, that is my 2 cents worth... Some may agree, some may disagree. But having studied real life occurances, and having played the role of both active shooter and of innocent bystander in these active shooter drills, I can speak firsthand about how these things go down.
Regards,
Raven6
Police, unless they are Military Police, ARE civilians. (Sorry, just a pet peeve.)