One (more) fallacy that shows up in these debates is that the world is comprised of two kinds of people, blackhearted villains and honest citizens. Of course, the world (and the human psyche) is much more complex than that, and there are good men and women who snap, flip, or degenerate into killers, whether it's because their synapses start misfiring, the lose their job/spouse/home/business/fortune, or they just have a couple of stiff drinks. We'll never be able to keep guns in the hands of the White Hats while denying them to the Black Hats, because of the uncountable shades of gray.
What we can do, however, is to limit how much ridiculous firepower one person can buy. Guns, more than anything, make killing people incredibly easy - they're fast, they're accurate, there's no physical confrontation required, there's a pretty simple learning curve, there's very little chance that they'll blow up in your hand, and they're not impractically expensive. No other method of killing people has all of those qualities, and that's not even counting in the cultural 'coolness' factor.
I just read some of today's articles on this shooter. He had an AR-15, with a drum. You'll have a hard time convincing me that any private citizen needs that for home defense or hunting, and I think a sportman's right to have that death machine available for entertainment can be foregone in favor of society's right to not have their midnight movie theaters shot up by deranged psychos.
If anything good comes out of these 12 deaths, I hope it's that.