Culture.
There are other countries with lots of guns with much lower gun murder rates than we have. Michael Moore makes a big deal of comparing the U.S. to Canada, which he claims has more guns per capita* but a far lower gun murder rate. He is puzzled by the reasons for this, but will not draw some obvious conclusions, because they leads to a number of places where few leftists dare to go.
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*Which makes it almost certainly not true, but we'll make the assumption for the sake of argument.