Here's a complex answer to your question; The American justice system is horribly flawed. Crimes committed against Americans by Americans are not always adjudicated properly and the appeals process and the capitalist lawyers work against the justice system in such a way as to exploit Constitutional rights in favor of the criminal by virtue of his or her humanity.
According to some moralistic view of liberal America, criminals, being human, reserve certain protected rights. As a result, criminals are spared the death penalty or life imprisonment even for the most heinous crimes. The result in a lot of cases is that these criminals are released on such terms as "good behavior, failed evidence, or the judgement of some left-leaning judge.
In significant numbers, these criminals are released back into society and because they have no prospects, return to crime. It is this criminal element and lack of law protection of the innocents that drives them to recomit crimes that cause harm to society. Because of this and because of our constitutional right of Life, Liberty and the Pursuit of happiness, our constitution provides us with the right to arm ourselves for our own protection. This we do with relish to preserve our livelihood and our families. Of course, the most efficient weapon against this criminal element are guns.