The trouble with opening fire at a rally to stop an active shooter is mainly the possibility of hitting innocents, by missing, over penetration, or by misidentification.
Add to that the possibility of someone else mistaking you for another active shooter and you see why there is hesitation to fire.
What to do? As they have shown many times if the active shooter is in the process of reloading that is the time to attack! And if you are not an excellent combat shot, then trying to engage such an active shooter in a crowded location with such as a J smith .38 snub, well that’s asking for big trouble.
If you can, take them down and disarm them. But like being an excellent shot, I strongly suggest you have some H2H skills so you can tackle and disarm such a shooter.
“To you who call yourselves ‘men of peace,’ I say, you are not safe without men of action by your side” Thucydides