The American gun laws are crazy. One would think by now they might have put two and two together and worked out that the frequent massacres are something to do with allowing all and sundry to own the weapons!
That's probably an element of it, but it's a more complicated situation than that - Switzerland, the Falkland Islands and Canada all have high rates of gun ownership (compared to, say, the UK) although not AS high, but they don't have gun deaths, be it accidental, homicidal or self-inflicted, or gun-related injuries at a rate even close to proportional.
The number of guns is part of the issue, but the American attitude towards guns and gun control is more of a problem - there are places in the world that quite adequately show you can have high gun ownership and moderate levels of gun violence and firearms related deaths, and then there's 'you'll get my gun when you pry it from my child's self-inflictedly cold, dead hand' America.
O.