One of my ancestors was a preacher on the frontier in the 1790s, and everyone brought their guns to church. Not a bad idea even today.
The decision when to shoot and (far more importantly, when NOT to shoot) is an extremely difficult one and usually must be made in a matter of seconds or less. Police officers go through elaborate training and are subject to stringent rules so as to not cause more harm than they are trying to prevent.
Have you judged the situation correctly (in a second or two)?
Can you shoot the perp without endangering others near or (and few think of this) behind the perp or behind the wall or window behind the perp?
Will it turn into panic shooting, a tragedy even police officers are sometimes subject to.
Will the fact that an innocent parishioner pulls his own gun in self-defense make him (and his family sitting next to him) instantly the primary target for the lunatic with the shotgun, machine pistol, or automatic or semi-automatic assault rifle?
Will the fact that many people are carrying concealed weapons into public places, like churches, libraries, train stations, and so on lead to shootings other than those defending oneself against the rare lunatic - such as the sort of argument that usually leads to shouting or fistfights suddenly turning into gunplay? In other words, people whose only intention for carrying weapons is to protect themselves can also be potential killers given the right provocation (many of them may be just as unbalanced as the guy who goes out intentionally to murder, not to mention drunk or drugged, or just pissed off about something or other).
Not only does carrying a handgun increase the odds that you may feel brave enough and powerful enough to intervene in a situation you might have avoided, but the ease of killing with a handgun lowers the trigger point (you should excuse the pun) that might lead to fatal violence. People are a lot less likely to let a situation escalate to fatal force if they have to get up close and personal with a knife or fists and such, putting themselves at risk. It is just too ‘easy’ and ‘safe’ and ‘sanitary’ to stand off at 10 feet and exert a couple pounds of pressure with a single finger.
This is not a simply issue, and even the police, with all their training, guidelines, experience, and discipline sometimes make tragic mistakes (not to mention the incidence of off-duty cops who get into fatal bar fights, shoot their spouses, or eat their own guns).
Please note I am not writing against gun ownership (I own a Colt .356 Magnum Python and an Walther PPK/S chambered for 9mm short). I am only trying to raise points that we should all consider before simply assuming that an armed public populace is inherently safer than an unarmed populace.
After all, even in the “Wild West” the first thing you had to do when you entered most towns was to check your weapons with the sheriff, to be picked up only when you left town.