I generally sit about 4 rows from the back, and 8 rows ahead of me. If someone with a drawn pistol at their side walks past me, he's either going to shoot or not in the next couple seconds. If that person was after a mass shooting, they could just stand in back and have at it.
There are a couple ushers that hang around in back, but generally that's it.
On stage there are generally 2 priests, a deacon or 2, and a herd of altar boys/girls.
I don't think I could justify taking a shot until the person with the gun made a move to raise their gun. Maybe they are deranged and wanted a shouting match, and brought the gun to reinforce their point that they are serious.
I would definitely push my family to the floor, or try to make for an exit.
If the person wanted a shouting match they would get tackled pretty quick. A lot of us big guys sit on the aisle seats (more leg room

I was primarily curious what the group thought as far as a shoot/no shoot scenario where the threat is walking away from you, but into a position to threaten everyone else. You have the drop on them, but they haven't made the move yet.