There are several reasons why America hosts most of the recent mass slaughterings. While such incidents can happen anywhere, they are most prevalent in free, or democratic countries. In autocratic or repressive regimes, mass killings by explosives are for political reasons - Chechen terrorism in the Moscow theatre bombing or the Beslan school massacre. But not random, mindless slaughter. There is no logical way to prevent such massacres in a democracy.
The gas-masked gunman who opened fire at a theatre full of people, killing twelve and injuring dozens more, reportedly had a shotgun, two pistols, an assault rifle, gas canisters, and potentially explosives in his home. What I don't understand is how it can possibly be alright for a civilian to have access to these kinds of weapons.