Im not sure about the ccw carrier that had opportunity to end the massacre not engaging. From the little i read on the post the crazy was dealing with a malfunctioning weapon at least for a moment two.
The decision not to engage in case of a miss and someone fleeting behind the nut job somehow doesnt make muster. I wasnt there so this arm chair quarter backing but, faced with a mass shooter intent on killing more people and offered a chance to stop him from likely killing several to many more innocents vs the chance I might miss and hit someone I saw MOVING behind him and quite possibly out of the way by the time I fired I believe I would engage.
As I say I wasnt there have limited knowledge of the whole shooting encounter but based on just that I dont think I could do nothing and retreat in that exact situation and allow the killing to continue.