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.