I'll put my Snark-o-meter on pause for this one since it's apparently been well-covered.
If I am shopping, I will not smash the display because all the guns here (as others have said) have trigger locks on them and I won't want to waste time fucking with one after making a ruckus that may draw the shooter's attention. If I'm an employee with the keys, I will certainly access whichever one I have ammo for.
If I am with my family, I will do my best to get them out of the store. If I am alone... I would probably still do that... but I would assess the situation first. And if I thought I had even half a chance at success, even if I was using a compound bow or baseball bat, I would do what I could to stop the threat.
A CCW does NOT mean you are a cop. You do NOT have an obligation to defend anyone but yourself. In many jurisdictions, the very people you save will testify as to the recklessness of your actions and say "Play stupid games, win stupid prizes." But I weigh that against the thought of watching the news report the deaths of a pregnant woman or a 6-year-old child or a wheelchair-bound WW2 veteran, killed by that shooter while they waited for "the professionals" to show up and save them. I would never be able to look in a mirror again. Sometimes, doing what's right is more important than doing what's smart.
Today is the 69th anniversary of the liberation of Auschwitz, btw. Different circumstances and different scale but a reminder of what can happen when good men do nothing.
Do something.