The logic is really very simple.
You’re in a crowd of a hundred people. Where, doesn’t matter. You could be at a convention, in a shopping mall, in a large restaurant, or on the sidewalk on a busy city streetcorner.
The logic is this: If none of those hundred people is carrying a gun, everybody is safer. If five of the hundred people are carrying guns, everybody is less safe. If fifty of them are carrying guns, you’re in a very dangerous situation indeed.
Why? Because in any group of a hundred people, it’s quite likely that two or three of them are either mentally ill, mentally impaired by alcohol, or just very, very angry. The more people in the group have guns, the more likely it is that one of them is going to pull out his or her gun and use it.
Logically, then, if you choose to carry a gun (a concealed handgun, or openly), the other 99 people in the crowd are less safe. I know that people sometimes claim they carry a concealed handgun out of concern for their personal safety. So the question is, do you care about the safety of the other 99 people in the crowd, or do you only care about your own safety, and the hell with everybody else? It’s really that simple.
Yes, there are good, logical reasons to carry a gun. Not very many reasons, perhaps, but there are some. If you’re a courier entrusted with large amounts of cash or important documents, you should carry a gun. If you’re a private detective, you should certainly be able to carry a gun when your own judgment indicates that it’s a good idea. If you’re being stalked by an ex-spouse or ex-lover, absolutely — carry a gun! If you have a meth lab in your basement, or even a marijuana plantation in your back yard, having a gun within easy reach makes good sense.
But those reasons don’t apply to very many people. Most of us have no reason at all to carry a gun. Yet lots of people who don’t need them do carry them. And why? It seems to me the reasons boil down to two: irrational fear, or an arrogant need to assert one’s individual rights and freedoms even at the expense of other people’s safety.
Please note: Nothing that I have said here has anything to do with your legal rights or the Second Amendment. I’m not concerned, at the moment, about what’s legal. I’m concerned only about public safety.
It’s often useful, in questions of personal conduct, to ask yourself, “What would the world be like if everybody did this? Would I want to live in that world?” If you honestly believe that the world would be a better, safer, and more joyous place if everybody carried guns, I hope you’ll consider, seriously and at length, the possibility that your emotions may have warped your rational judgment.
I like freedom too. But we all have to live in the world together. Sometimes — quite often, in fact — our individual freedom has to be tempered by the understanding of how our actions may affect others.
Jim Aikin's Oblong Blob 
If you consider any venue where there is a shooter locked and loaded and looking for targets it doesn’t matter whether you’re armed or not, that is unless you too are locked and loaded and scanning for threats. You do not have enough time to unholster and cock a pistol and acquire a target that is already poised to shot. You are dead. And let jus presume that everyone in your described crowd is only packing as a defense measure then there is never a threat. So in the latter scenario no one needs to be armed because there are no threats. In the former your carrying of a weapon is irrelevant unless you are prepared for an impromptu and immediate firefight. Holstered is like having no weapon at all in this case.
All very true, Ron … but I’m not sure the categories of “looking for targets” and “only packing as a defensive measure” are black-and-white. I’m pretty sure there are a few disturbed individuals running around town who would tell you in all sincerity that they’re only packing as a defensive measure, when in reality they’re bundling a lot of suppressed rage that could easily erupt if they _perceive_ what they feel is a threat. Such as, you know, somebody bumping into them. That being the case, I don’t think your “latter scenario” can be relied on ever to happen.
With respect to the former scenario, you’re not necessarily dead, because the maniac might be aiming at somebody else. But in a crowd where there are, let’s say, half a dozen people with guns, five of whom are sane and one of whom starts shooting, there will be massive confusion. You may whip out your gun and take aim at the maniac — and someone else with a gun may jump to the conclusion that you’re the maniac, in which case you’re dead. The more guns there are in the crowd, the greater the potential for an eruption of lethal confusion.