What could I do in the face of a mass shooter? I don't own a gun. I've never even fired one. The idea that I could out-shoot a committed killer is a myth anyway. And while I'm big and strong at 6-foot-3 and 200 pounds, I'm not quick on my feet. I can't dodge a bullet, but I can't wrestle one either.Just as irrational as the distinction between "likely" and "unlikely" is the distinction between "difficult" and "impossible." You've reasoned that in the unlikely event of a shooter, it is impossible that you could do anything about it. Experience and evidence shows that this is wrong, and furthermore, that it's the way out of your problem.
We take precautions against many dangers more remote than encountering an active shooter. Sometimes it only makes sense to do this if we wrap a bunch of them together, so that the probability begins to justify the expense of the precaution. I have a Homeowner's Insurance policy that exists to manage a bundle of many unlikely (but expensive) dangers associated with being a homeowner. It would be silly to buy a policy for any of those risks by itself, as all of them are quite unlikely. Taken together, though, they justify the minor annual expense of purchasing the policy.
So if you don't want to carry a gun and learn to use it accurately, OK. You're free to make that choice. But consider bundling the active shooter threat with a number of health-related threats associated with being a big guy who isn't "quick on his feet." Join a jujitsu club, or take Krav Maga, or something similar. The physical exercise will manage a bunch of other threats, and you'll also develop a much increased capacity to escape in the event of an active shooter -- perhaps even to overcome and triumph, if you happen to be in just the right place at the right time.
Like those guys on the French train. Heroes, we say, but the day before they were heroes they were just guys on vacation. American guys. Guys like you, if you choose to be like them.