It isn't guns, per se, that are the problem, although easy access to weapons of war has become a problem of its own.
The actual problem is that we, that is us "Americans", hold a particularly dangerous idea sacred, and that idea is that when we are outraged or angered to a certain degree, it's OK, or even admirable to "go Postal", and that violence is a legitimate last resort when things don't go the way we want them to.
This is an idea that we reinforce and restate often in films, on television, in video games, on talk radio, in internet discussion forums, and in casual conversation, and even in political dialogue (not so much in books, and hardly ever in poetry, tho).
Most of us know better than to shoot anyone with our 300 million odd weapons, and just express the "nuke em" response out of annoyance or frustration, but there are some among us, including mentally and emotionally distressed, socially and economically alienated, and otherwise marginalized individuals, who act it out.
When we see Tony Soprano or Walter White commit mayhem on their antagonists, we feel satisfied and relieved that "frontier justice" has finally been done, but when someone walks into a theater, or a school, or church, we shake our heads and wonder why.
The thing is, more and more, the behavior is moving from the margins to the mainstream, and from law enforcement to foreign military action, the "last resort" has become the "first response".