All people have God-given rights. Several are enumerated in the Declaration of Independence. You are probably familiar with the unalienable rights of "life, liberty, and the pursuit of happiness." But this organic charter also states that "whenever any form of government becomes destructive of these ends, it is the right of the people to alter or abolish it, and institute new government."
This human right to abolish existing government and establish new government is usually met with force. Witness the exercise of this fundamental human right with varying degrees of success and violent resistance in nations such as Egypt, Libya, Syria and Israel. Do I even need to state the obvious - that prosecution of these democratic movements would be doomed but for weapons in the hands of the average citizen? And let me tell you, a Syrian with a hand gun or shot gun is not going to make a dent against more capable weapons.
All of history tells us any unarmed (or out armed) citizenry is doomed to subjugation, persecution and even extermination by the state. In our own lifetime consider the atrocities in Darfur, Bosnia, 1940s Germany, China, the former Soviet Union and Cambodia. The lesson being that as Christianity and the value of human life and dignity are diminished by a culture, the state naturally becomes the greatest perpetrator of violence upon its own people, all under the auspice of advancing the good of the state.
Those who prefer not to accept these undeniable truths, but prefer to disarm decent people like you and me, are actually condemning future generations to collective violence that eclipses the acts of any lone madman.