I voted other, and put in the blank cultural. Since the days of Cain and Abel, violence has been the one part of humankind which has linked us all. Or for those who wish, you could say that violence is in our DNA. And that part of what makes us the dominant species on Earth is our willingness to dominate not only every species on Earth, but each other. Food chain and all that.
To say that it is only here in the U.S. that violence is a problem is of course, false and couldn't be further from the truth if one tried. Many other countries experience this today as well. Guns, poverty, the criminal class, and the entertainment industries are perhaps contributing factors, but there are many who would say that we should return to G-d as a nation, to bring about peace among ourselves. As a Christian myself, it's tempting to take that route, but the fact is, there will always be those who have what we used to call a short fuse, or a violent streak. It is just in the last generation or so that things have become more widespread, not in the rate in which they happen, but in the rate in which we see them reported, with the 24 hour news cycle.
I chose "Other", and commented that there are many people who are just plain evil, regardless of genetics, environment, finances, or family influence. I don't think there needs to be a "cause".
I'd love to see what the "Other" choice contains. I chose "Other" and entered "government" as my candidate. I have ample evidence to back that up. If you look at each of the choices listed, for most you can make a strong case that government has either created or exacerbated these factors. That's why I have taken to tagging all my Facebook postings with the derivative "Furthermore, I consider that government must be destroyed".