I think there are two parts to that: 1) Is the good guy there? 2) Did he do anything?
Here's an analysis of FBI data (2000-2017) saying that armed citizens are present at active shooter events around 11% of the time. When they are present, they are very effective. Armed Citizens Are Successful 94% Of The Time At Active Shooter Events
acronym 7/20/2022 8:49 AM