No discussion yet? Wow...
Under "what would I do", I don't have a one-size-fits-all-situations answer, but certainly the example of Dan McKown is ever before me. Dan intervened without using his handgun in the Tacoma Mall shooting, and paid the price of a lifelong serious disability as a result.
I don't have time to go and find all the original reporting, so take the following with a certain degree of caution--but I don't recall Dan ever saying in print that he held back because he couldn't safely take a shot, but rather because he was taken aback by the youthful appearance of the shooter.
My takeaway message from that is that if you are going to intervene, do so decisively.