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The only thing I can say, while I agree with you, is that we HOPE they don't just shoot up the place, unprovoked. There's a good chance they won't, but there's always that chance

You are right, in most cases they won't. If they do the decision matrix is complex on how to react and you have a split second to decide what to do. The more shooters you have the more complex it gets.
This is not meant to be an exhaustive list by any means. And I will stress there is nothing wrong with keeping your weapon concealed and remain a witness. A CHL holder is not expected to risk his/her life and attempt to take on a multi shooter event.
In considering to act the first consideration Is your own resources. What weapon do you have, and how many rounds?
After that in no specific order, as each of these could be done first depending on circumstance:
Do you,
1- take out the leader?
2- take out the most powerful weapon?
3- take out the perp nearest you?
4- what do you do if a suspect takes a hostage?
5- if forced do you give up your weapon?
6- how do you handle other CHL's? Are they CHL or other BG?

That is just a few of the decisions you have a split second to make while the police, DA, and grand jury will have all the time they need to decide if you were right.
There are many other decisions that you will be making in a split second of time based on the reactions of the suspects, other customers.
You can bet your bottom dollar that if a customer dies, regardless of if you shot them or not, some family is going to try to sue you because some bottom feeder lawyer will convince them you were at least partially responsible for their loved ones death.

Why not just be a good witness? Here are some facts:
1- unless you train for this scenario, or have been trained, the odds of a proper reaction are about 10%. Numerous studies show 90% of the public will freeze or panic.

2- some os you said you would open fire. Why? There were six of them, odds are you would have instigated a massacre.

3-I am a trained, certified SWAT sniper (retired), even I would just be a witness unless the suspects fired first. Police tactics 101, confront the robbers outside. This way you minimize the odds of a hostage situation and bystander injuries.

4- all the true heros are dead. Don't be in a hurry to join them. Your eyes and ears are truly the best weapons you have.

Until you are involved in a shooting scenario you really don't know how you will react. Pray daily you will never find out.