The Positive Gun News thread is full of examples of what stops bad guys with guns. And often it is a good guy with a gun.
If that were true, the Positive Gun News thread would not exist, because it gives examples of actually stopping bad guys (some with guns) and is not proposing gun control.
So, often, a bad guy with a gun is stopped without a good guy with a gun, and, often, a good guy with a gun isn’t a way to stop a bad guy with a gun, and, often, a good guy with a gun is a way to stop a bad guy, sometimes with a gun and sometimes without.