It would make them look weak and impotent if did all this planning and preparation only to be stopped by a couple of citizens with CCW’s.
[It would also make the gun grabbers look stupid, but that's a different story]
Better for them to select a target where the people have already been disarmed by kind and benevolent government. /sarc.
I don’t get the logic in your post: How many people pack heat before going to church? I don’t think very many, if any at all.