It begins (for the sake of brevity) with the English Bill of Rights in 1689 [see here]. This bill limited the power of the monarch, instituted elections and allowed people to arm themselves. Fast forward 100 years to 1789 and the philosophy of John Locke continues its lineage through to the US Bill of Rights, the amendments to the Constitution. It is here that we see our old friend, the right to bear arms. This right is not, as in the English version, granted from government, it is guaranteed to the individual away from any government interference.
Which brings us to today. The civil society can change, but the universal rights inherent in being human does not. How could it? It is worth keeping in mind that we in the West have enjoyed this right that governments cannot take away for well over 300 years. Tragedies will always be with us as we are imperfect and no attempts to curtail evil can ever be 100% effective. But no tragedy should be allowed to infringe on the most basic of rights: the right to defend your life from someone who may want to take it.