After the tragic events at Virginia Tech, there seems to be a right wing notion that a good way to suppress violence or crime is to allow or force a population, (city, college, community) to arm itself. The simple theory is that a responsible armed populace would be able to both defend itself, and act as a deterrent to armed criminals intent on robbery or inflicting injury on the public.
Let's take a look at how this might play out. In a good size lecture hall, you might have one or two hundred students in some basic course, let's say, chem 1a. Everyone is packing and feeling safer for it. Professor Jones has an AK47 behind the lectern. Amanda, who sits in the front row, has a 357 magnum. Pete sits in the back of the hall so he won't be called upon, but he has a 9mm with an expanded clip.
Let's pretend that I am a deeply disturbed individual who is a loner and harbors a general anger towards the public for perceived slights to my person.
In this scenario, I don't have to go postal or expose myself at all. I phone in some threat to the local police and wait until they are almost at the campus. I've rigged a few firecrackers in several different halls and set them to go off about the same time. The firecrackers go off in the chemistry lecture. Professor Jones rams home the clip in into his AK.
Pete reaches for his 9mm, but fingers the trigger and fires a round through his jacket into a student next to him. Suddenly, there are three pistols firing at him and he is hit by one round. Two of the rounds aimed at Pete miss and hit other students.
A round ricochets off the chair near Amanda and she squeezes off 4 shots blindly in the direction from which she thought they came. One hits another student, and the rest miss. Raoul is grazed and opens up with his assault rifle. He kills 5 and shatters the lecture hall window.
The police arrive to multiple shots fired, multiple victims, and a scene where they cannot tell terrorist from innocent civilian. However, if someone turns their way with a weapon, they will fire in self defense. The police SWAT team arrives with the good stuff. All of a sudden, the class has become a free fire zone. All of a sudden, this new situation has the potential to make the casualties at VT look trivial. Does anyone think the police won't respond if someone carrying a gun turns towards them? The VT students reported that police on the scence made them put their hands on their heads because they did not know who was posing the threat.
If anyone thinks that a classroom of students will necessarily be well versed in use of weaponry and in legitimate target acquisition, they are wrong. I posit that the population packing heat and thinking they are qualified to do so is about like our population of drivers. Most think they are competent, but are not equiped to handle dire circumstances. While many gun owners know how to safely conduct themselves under normal circumstances, in a large armed group, there are inevitably a fair number that should not be allowed to touch a weapon. And in a free fire zone, who knows how badly all the rules would fly out the window. Your disturbed individual wouldn't even have to fire a shot.