If you're the biblically minded sort, then the trouble began when a jealous Cain clubbed Abel to death, but if you're evolutionarily minded, then it's a 'chicken and egg' question. Violence had no beginning, except perhaps in the Big Bang, it was always here, coded into the DNA. If people are just grown-up animals, more articulate versions of the creatures who eat each other's young, and sometimes their own young, there is as much use in wondering about the nature of evil as there is in trying to understand why a killer whale kills.
But debating how many devils can dance on the head of a pinhead is largely useless. We are not a particularly violent society. We are a society sheltered from violence. No one in Rwanda spends a great deal of time wondering what kind of man would murder children. They probably live next door to him. For that matter, if your neighborhood is diverse enough, you might be unfortunate enough to live next door to any number of war criminals, all the way from Eastern Europe to Asia to Africa.
The issue isn't really guns. Guns are how we misspell evil. Guns are how we avoid talking about the ugly realities of human nature while building sandcastles on the shores of utopia.
The obsession with guns, rather than machetes, stone clubs, crossbows or that impressive weapon of mass death, the longbow (just ask anyone on the French side of the Battle of Agincourt) is really the obsession with human agency. It's not about the fear of what one motivated maniac can do in a crowded place, but about the precariousness of social control that the killing sprees imply.