I feel like there are two separate issues here. What is legal and what is practical/advisable/smart.
Obviously an individual’s state law is going to determine what is allowable or not in that state. I would be surprised if a state law does not allow for the use of deadly force in the defense of others. Of course I do not know, but most of the self-defense laws I have seen for many states include some aspect of “in defense of another”. Here in California, our laws do include “in defense of another” and there is no duty to retreat.
Now, I have not heard of any state that has laws which would obligate a concealed carrier to act and intervene. Maybe that exists somewhere, but again I have never heard that.
Practically speaking, you are taking a big risk by intervening. What if you engage and shoot an innocent person? Are you witnessing what is truly an active shooter situation, or a quarrel or fight between a small group of people (like rival gang members).
Me personally? I would let the individual situation I find myself in dictate my actions. I might flee to safety, I might intervene. I am confident in my state either action would be lawful.
I bet I can create scenario after scenario that would cause even the most hesitant of those here to act and engage an active shooter. Each and every situation and detail within it (the totality of the facts) will be the driving force in those decisions.