The traditional wisdom is that carrying a gun for safety doesn't help in such sudden events, as the "bad guys" are generally more prepared, have the initiative, etc.
This is obviously different for instance in a war, where both parties are more-or-less prepared to use a gun at any given time.
Now, I've had military training (somewhat mandatory in my country), and do own a gun. But that is because I find target practice and hunting trips interesting, not because I would find any safety application to owning one.
For instance, I went motorcycling through Caucasus right after the revolution in Georgia. This was a time when gas station attendants carried Kalashnikovs and people customarily brought guns to bar, but still I would have never considered taking any weapons there, as those would have only escalated any problem situation in a bad way. And obviously in the end there were no problems. Nice and hospitable people there, and a beautiful country to visit :-)
In particular in crowded events: one worry is that civilians playing hero will just end up shooting more innocent people, as they try to shoot at this guy in a crowded theatre and hit someone else.
Shooters of this sort being confronted by citizens who carry guns has happened several times in the past, and in no cases I'm aware of has anything even remotely similar to this super hypothetical come to pass.
In more general incidents, the numbers I can find suggest that about 2% of armed-citizen-response-to-crime shootings shoot an innocent person. Mostly it seems actually pro-gun advocates quote this, as 2% is supposed to be a lower figure than the police's error rate.
This article is heavily slanted and calling the sound judgment of Zamudio in the events described nothing more than "sheer luck" is patently insulting at best and fear mongering at worst.
Also, your second paragraph is somewhat ambiguous, I'm not sure if you mean to imply that the incident described is the only incidence of a confrontation with a deranged shooter. That is most definitely not the case. There are many examples of this happening:
The characterization of it as "luck" is Zamudio's, who referred to it as such repeatedly, in multiple interviews on multiple occasions, not my own, so I fail to see how it could possibly be insulting to call it "luck". He seems to believe that only luck kept him from making a significant mistake, and he can imagine it just as easily having come out the other way in a split-second wrong reaction.
Also, an armed citizen if far more likely to suffer consequences if they shoot the wrong person by mistake. And the training they get is going to be much more "Don't shoot unless you're really sure" rather than "shoot first to protect yourself".
And I would expect thee preparedness of the average concealed carry permit holder to decline if we do more to reshape the US around the notion that an armed society is a polite society.
You are relying on anecdotes. However, given that shooters of this sort are quite rare, is there even enough data to reach statistically significant conclusions about the risks from people returning fire?
In any case, if we were to follow this argument further, we would expect that many more citizens would be armed, which would likely change things. Given the inherent confusion of such a situation, I would expect we would start seeing armed citizens accidentally shooting other armed citizens because they misread the situation.
Even trained, battle seasoned soldiers make mistakes.
I'm much more concerned about the impact higher carry rates would have in relatively more common situations, like disagreements between motorists, etc.
Many people with guns feel empowered to double down in situations they would otherwise walk away from, and the involvement of a gun adds to the opportunities for a deadly outcome. Some gun advocates argue that an armed society is a polite society. I have my doubts about that, but I don't doubt that the consequences of impoliteness would be much more extreme.
This is obviously different for instance in a war, where both parties are more-or-less prepared to use a gun at any given time.
Now, I've had military training (somewhat mandatory in my country), and do own a gun. But that is because I find target practice and hunting trips interesting, not because I would find any safety application to owning one.
For instance, I went motorcycling through Caucasus right after the revolution in Georgia. This was a time when gas station attendants carried Kalashnikovs and people customarily brought guns to bar, but still I would have never considered taking any weapons there, as those would have only escalated any problem situation in a bad way. And obviously in the end there were no problems. Nice and hospitable people there, and a beautiful country to visit :-)