One thing the public at large is ignorant of is how cops are now being trained to deal with the active shooter.
The "active shooter" is actually a recent phenomenon (recent meaning in the last 20 years or so). In the past people like that would shoot and run or shoot and take hostages and the first officer to arrive on scene would secure the perimeter and wait for the swat team to equip and show up and so on.
Now these shooters are shooting so as to kill as many as they can. Modern training dictates the first officer(s) on the scene not hesitate and grab their shotgun/AR15 and go in and hunt the sob down. No more "freeze police", it's find them and shoot them, in the back if need be.
I'm not kidding, I've been trained with this in in-service for the past 15+ years now. I truly believe if the media knew what's now being trained they'd go ape feces!!! But it's the right thing to do.
And armed civilians may need to do the same. An active shooter in a public place is there to shoot as many people as possible and the only thing that will stop them is another bullet. By the time any law enforcement or armed security (yeah, right! ) show up, dozens of people could be injured or dead.
Don't be rash but do take calculated risks. If you can, take the shot. An active shooter is like the Terminator. It can't be reasoned with, it doesn't give a schmit, and it knows nothing of mercy or remorse. And it will not stop...EVER! Until it is out of bullets or it is dead. Make the latter one the cause of it's cessation.