I'm a uniformed cop who works a walking beat in a high school. I can't speak from the mindset of a non-sworn citizen, but I can speak from the mindset of a street-level cop.
Let me first say that I have never had any problem with a law-abiding citizen carrying a gun. Not the first problem, and in this very gun-friendly state, I assume that about half the people I talk to are either carrying, or have a gun within arms reach. SO, let me say this again; I have never had a single problem with a law-abiding citizen carrying a gun.
Now, lets go to the active shooter. If I encounter an active shooter I am going to engage the shooter. I will continue to engage the shooter until the fight is over. If I have time (while running toward the fight), I'll call my cavalry on my radio, to get them rolling, but my intent is to engage the shooter and end the fight.
If you are a plainclothes citizen, when I get to the fight, I expect you to clear my field of fire. The safest way for all concerned is for you to immediately go prone. I won't have time to short the sheep from the goats, and anyone standing up holding a gun will be engaged. I will continue to move toward the shooter, to attempt to end the fight.
If, at the end of the fight, I'm still standing, I will handcuff the shooter, regardless of his medical condition. If my cavalry has not yet arrived I will probably also handcuff anyone holding a gun. Don't take it personally, I certainly won't. I'll apologize after it's over, and if it turns out that you were trying to help, I'll see that you're given appropriate accolades. Tell reporters what a great guy you were, see if the Mayor will give you the keys to the city.
This is an event, that while I hope it never happens, I've trained for, studied, wargamed, and had long, earnest discussions with like-minded souls since 2002. Lets hope I never have to use all that training.