Whenever there’s a violent tragedy, someone immediately starts using it as an excuse to restrict civil liberties. Many on the left understand this when it comes to the Patriot Act, but not when it comes to gun control. (Conservatives have selective blindness in the opposite direction.)
In deontological terms, the right to self-defense is the foundation and presupposition of all other rights; and forbidding private citizens to own guns while allowing police and soldiers to carry them is a violation of moral equality – a reserving of weapons to the powerful while denying them to the powerless.
In consequentialist terms, gun control is a perfect example of the broken window fallacy. Deaths caused by gun use are seen, because they happen. Deaths prevented by gun use are not seen, because they don’t happen. (By “gun use” I mean not just firings but also mere brandishings.) First, preventions are underreported (since few are eager to be victimised twice – first by a freelance attacker and second by the cops), and second, when they are reported, they’re not exciting enough to get much publicity.
People who favour stronger gun control laws focus on the deaths they hope to prevent, but rarely consider the deaths their laws would cause. One useful corrective to this attitude is the Cato Institute study Tough Targets: When Criminals Face Armed Resistance From Citizens.
And although I’m not a fan of the NRA (which I don’t regard as an anti-gun-control organisation), they have a useful blog, The Armed Citizen, documenting lives saved by private gun use.