I've said it before and I'll say it again...
The problem with "What stops a bad guy with a gun? A good guy with a gun" is that is takes someone getting shot first to determine which is which.
Here is a story where even if the father of this young girl had a gun and was able to shoot back at his daughter's killer, she would still be dead.
Police say suspect confesses to road rage shooting that killed New Mexico 4-year-old
And here is another story where a good guy with a gun mistook another good guy with a gun as a bad guy with a gun. Chances are, if the dead person in this story did not have a gun, he would still be alive today.
Florida Black Man Shot and Killed By Plainclothes Police Officer
Must I go on?