Given the situation described in the first post - man gunning down his wife and unknown male/possible friend. I kind of fail to see how an armed civilian would be able to do much good.
What if that armed citizen happened to be the intended victim? He would have been right there in the middle of things as they went down, and there may have been a possibility that he could have responded. You seem to be thinking of uninvolved citizens rolling in to the rescue in the middle of an event. That is NOT what I train for. That is a job for the police. I hate to inform you, but if I were to see someone attacking you from 50 feet away, I most certainly would NOT draw my firearm and attempt a head shot on the bad guy to save you. That seems to be a fantasy held by non-gun people of how gun people would react. At the point of your attack, my priorities would be to protect my family, myself, my friends, other bystanders, and then you in very last place. Chances are high I would not get around to the "protect you" part. That sounds harsh. But you are not my priority. I am not in law enforcement and I do not train to actively insert myself into a violent situation. I train to get my family and myself OUT of a violent situation. You only get to ride along on my coattails on the way out if doing so doesn't involve putting my family and myself in additional danger. Once my family was totally out of danger, then and only then would I consider other things I could do to permanently put an end to the violent situation (and no, that wouldn't include any headshot attempts from 50 feet away).
And anyway, are you the kind of person that could live with that for the rest of your life?
It would be hard to live with, yes. But it would be harder for me to live with having cowered in place, totally submissive to the bad guys, simply watching my family, friends, and bystanders being murdered.
Another situation referred to here - Charlie Hebdo. Two men rush an office room with about a dozen people inside, including a LEO specifically tasked with protecting them, probably a highly trained individual. We all know how that turned out.
So, if you cannot guarantee that you will win each and every time, that means you shouldn't even try. Is that what you are saying?