Another maybe odd view I have is that the rule of nature is that there are predators and prey. The prey can flee, try to hide or fight back. If fleeing or hiding is not possible or known to likely fail, the prey should fight against the odds.
I can not know what I would do in the future as no one knows the future. But in the past, anytime I was being put into the "prey" situation my goal was to do anything to shift my role to also be prey. I have to first flee or hide - but my goal is to shift roles - something predators rarely consider when the are operating in the predator mode. They do not expect the prey to attack, do not expect the prey to double back at them, do not expect the prey they are chasing and now trying to hide - instead be lying in waiting.
Most mass shooters are not combat experienced and have very little firearm experience. They do not grasp out quickly a person can dash 30, 40, 50 feet. They do not realize their vision only allows them seeing less than 50% of what is around them. They think horizontally and 2 dimensional. They do not grasp that almost anything hard is a weapon. They do not understand natural delay times in reaction to being attacked. They do not realize that actually, in the final outcome, maybe it wasn't his gun, but the gun of his prey who got it away from him, killing him with his own gun.
If you actually read about mass shooting in settings where their prey (other people) are close, when any DO decide to attack rather than flee, more times than not they have been able to overcome and disarm the shooter.
That even now is in most training for mass shooting situations. Flee. Barricade. Hide. If that fails? FIGHT with anything and everything you can find. Even if shot while charging, that doesn't mean you die and doesn't mean he wins. It is not like Hollywood where if someone is shot they instantly fly backwards dead before they hit the ground.
Most people don't realize in nearly any indoor setting they are surrounded by weapons - mostly blunt force and throwable weapons. Being combat inexperienced, the shooter would instinctively first duck IF he sees it. There is A LOT of psychology involved in violent conflicts, including subconscious instinctive reactions.
These lone wolf, shooting and fighting inexperienced shooters are already so emotionally charged up having to deal with massive never experienced info they are highly prone to have confused and slow reactions - plus guns are extremely loud, which is highly disorienting - which there are always breaks in their shooting heard if it is recorded by audio. Having ear protectors or not, their hearing is highly reduced - still another disorienting handicap and vulnerability (why police use flash-bang devices).