If I was carrying and there was an active shooter, I sure as hell would run or hide before fighting.

You don’t know who the active shooter actually is. Maybe the guy you saw with a gun is a plainclothes or off duty cop who is responding to the actual active shooter. Maybe there is more than one shooter, and confronting the one you see makes you a target for the one you don’t. Maybe the cops find you after shooting the active shooter, and assume you are the perpetrator.

For clarification, I don’t carry a gun, I just used myself as an example to simplify the text.