Faith and proficiency are two different things. Shooting, all by itself, it a very perishable skill; Gunfighting even more so.
I don't doubt that there are good-hearted folks with service history, but how many of them have stayed sharp? That takes consistent and deliberate effort.
Add to the mix: a few genuine bad guys start doing the nasty, a random selection of average Joes and Janes happen to be packing in the area, and now there is a crowd of folks shooting at a crowd of folks. I'm gonna bet that the Blue-on-Blue, random missed shots due to lack of training, cardiac events, and responding LE shooting well intentioned good guys with guns, will make the general chaos and body count grow exponentially...
By way of example: I'm several months into my beard right now; I look like a mountain man with a high-and-tight. If I'm wearing a wool watch cap and a heavy coat (it's Winter), how much different from your run-of-the-mill Tango do I look? If you see me (justifiably) returning fire on bad guys, and have a chance to ambush me (because I "look" like a terrorist), whatcha gonna do hero? What am I gonna do if I realize you are shooting at me and you're not in a uniform? Do you look like a valid target to me? I burn through ALOT of ammo each month; do you want to be on my receiving end? What are the LEO's gonna do when they show up??? (Hint: it's shoot everybody with a gun that isn't them)