The good guy with the gun does in fact make a difference.
But according to the Crime Prevention Research Center, they make a much bigger difference than even I thought:
Evidence compiled by the Crime Prevention Research Center shows that the sources the media relied on undercounted the number of instances in which armed citizens have thwarted such attacks by an order of more than ten, saving untold numbers of lives. Of course, law-abiding citizens stopping these attacks are not rare. What is rare is national news coverage of those incidents. Although those many news stories about the Greenwood shooting also suggested that the defensive use of guns might endanger others, there is no evidence that these acts have harmed innocent victims.
Part of the problem is that the FBI is a little sluggish about counting episodes where the spree killer commits suicide after being confronted; the two events are not separate when they are inextricably linked.
But some of it is just plain bureaucratic dishonest. One fairly bald-faced example:
For example, the Bureau’s report about the Dec. 29, 2019 attack on the West Freeway Church of Christ in White Settlement, Texas, that left two men dead does not list this as an incident of “civic engagement.” Instead, the FBI lists this attack as being stopped by a security guard. A parishioner, who had volunteered to provide security during worship, fatally shot the perpetrator. That man, Jack Wilson, told Dr. John Lott that he was not a security professional. He said that 19 to 20 members of the congregation were armed that day, and they didn’t even keep track of who was carrying a concealed weapon.
Coverage of the actual episode right here. The FBI also treated this similar shooting as a “security guard” incident.