I've lived in the States my whole life and I don't fully understand it either. I think a lot of it is based on a nugget of truth, a huge helping of myth/wishful thinking, a bucket-full of manipulation, and big, stinky fear.
A couple things I feel about gun ownership in the U.S.
#1. While I can't speak for others in this situation, based on the times where something really scary has happened around here and the police were called... if there WERE a situation with an unstable killer, if we were to wait on the police, unarmed... we'd be dead by the time they got here.
#2. "People shouldn't be afraid of the government. The government should be afraid of the people." We are already on very shaky ground in the U.S. Many think that another civil war, workers' revolt, etc is inevitable, given enough time. Heck, it may even become necessary someday. Taking away the right to bear arms would have nothing to do with public safety. The massacres make for convenient excuses, but ultimately that's not why it would be happening. Just like the bill proposed that would give the government carte blanche to log and sift through 100% of everyone's internet activities was given the much more innocent-defending label of, "The Protecting Children from Internet Pornographers Act of 2011".
#3. A gun ban will only take the guns away from law-abiding citizens who are no threat to anybody. Those who want to pull this kind of heinous crap will simply go to the black market instead, just like drugs. And illegal guns make for a much bigger headache for forensics than legal ones.
Frankly, even if guns get taken away, I'll STILL be more interested in looking for ways to prevent people from reaching such disastrous ends in the first place. The current build of Reality is horridly broken. It shouldn't be a shocker that people are losing their sanity as a result.