Ed. I knew you were stupid, but you are even more stupid than I imagined.
The reason civilians have not stopped the most famous mass shootings is that they took place in locations where civilians were NOT allowed to carry guns. In other words, the bad guy was the ONLY person who had a gun!
Does that make sense to you?
What you said is equivalent to saying, “Lifeguards have not prevented a single drowning in places where there were no lifeguards. Therefore, we shouldn’t have lifeguards anywhere.”
You make a good point. Most mass shootings occur in gun free zones where armed citizens are not allowed.
Here are some incidents of mass killings stopped by armed citizens:
http://gunwatch.blogspot.com/2012/12/mass-killings-stopped-by-armed-citizens.html
There are 15 incidents with links to sources, listed. Most are pretty clear, some more than others.