There are some cogent arguments against restricting access to firearms. People enjoy guns for sport and hunting. The Second Amendment permits the private ownership of guns (though I doubt that the need for a "well-regulated militia" envisioned by the framers of the Constitution translates into such easy access to assault weapons or the need for multiple guns in a house with children or adolescents.) But there is nothing to say that more stringent gun control laws that could lessen the frequency of such horrible crimes cannot be judiciously balanced with constitutional rights.
But the Christian outlook on this has less to do with self-defense and more to do with the defense of the other person. Jesus asks us to love our enemies, not to murder them; to pray for them, not to take vengeance; and he commends the peacemakers among us, not those advocating for more and more and more weapons.
Was Jesus naïve? I wonder. I often marvel how some Christians can say that in one breath, and proclaim him as the Son of God in the next. Apparently, some believe that the Second Person of the Trinity didn't know what he was talking about. But Jesus lived in a violent time himself, under the heel of Roman rule in an occupied land, when human life was seen as cheap. Jesus witnessed violence and was himself the victim of violence--the most famous person to suffer the death penalty. It was not only divine inspiration but also human experience that led him to say: Blessed are the peacemakers.