“In not a single case was the killing stopped by a civilian using a gun.”
One cannot prove an ex post facto negative hypothesis.
First, they’re not making the logical claim that it’s impossible, they’re saying that so far, none has been. Which is logically correct, within the context of their statement: In order for a mass shooting to even have occurred, more than four fatalities need to happen, which excludes Pearl High and the New Life Church and the other cases. Plus, they are saying “by a civilian *using* a gun, by which they mean, by shooting the shooter. By those meanings, they are correct, although it’s highly dishonest of them to not count those shootings stopped because a gun was pointed but not fired, or those killings that were interrupted before the fatality count passed three, or, worst of all, by counting those killings that happened where armed citizens were banned.
The recent mall shooting in Portland Oregon was stopped when a civilian pulled out his concealed carry and pointed it at the perp, who then decided to take his own life.