Giving everyone guns so that they can "defend" themselves is a retarded idea. March explained it quite nicely, but there are more reasons for its idiocy, and that is that people without proper training using guns is asking for more and more bloodbaths; idiots fuelled by action heroes trying to save the day will do more harm than good, as I doubt _any_ of them will know _anything_ about their gun, other than that it (supposedly) kills when you aim and shoot.
In an unprepared situation, where a potentially low-intellect, suicidal psycho has gone apeshit in a close area you will not be prepared to take him down unless you have been trained to do so. Guns are made for killing, but the human mind is not ready to cope with extreme situations unless trained to do so, or properly prepared, or unless you grew up in a very, very bad place - think middle of the jungle with all the wild animals and psycho bushmen bad. You'll just end up shitting yourself and blind-firing around a corner, and then getting yourself killed when the other guy knows where you are and realises that his weapon is three times more powerful.
Everybody always speaks about handling guns and "taking down" the bad guys like it's a videogame, or a movie; it's not, and you will be shitting your pants before you get anything heroic done.
It's really not that hard to ban (mostly military) guns which are made for killing other people. Ban Sniper Rifles, SMG's, Assault Rifles and all that crap. You cannot tell me that you use an M16 to hunt deer in a city; the USA ought to get a law in place which forbids you to carry or own guns without a license (following extensive and expensive training) and limiting those licensed guns to simple handguns.
That being said, it will probably be impossible to pass such a law anyway, when all the conservatives will cry about their rights and how their freedom is being taken away, all the while lovingly taking it up the ass by the patriot act. Outdated laws can and should be changed; this isn't the Wild West anymore.
And now that I got that off my chest: every American I have met so far has been absolutely great, and I thoroughly believe that the stereotypical American ("Yeehaw" - I sleep with a loaded gun under by pillow" is quite rare, but in a country of 300 million there are still plenty of imbred lunatics running around, and regulation becomes essential.