Hugely complicated question. Two obvious factors are how easy it is to acquire tools of violence, and the self-regulated—and how do you suppose they do that?—underground economy of drugs. Of course US cultural history, practically from the moment whites arrived, has valued violence and conquest as the road to riches and ordering society. From dumping tea, to rebelling against the crown, to massacering the original inhabitants to take their land, to fighting one of the bloodiest wars ever to decide whether slavery (moreviolence) should prevail, to the KKK, and Timothy McVeigh, there's a strain in society that sees violent, or outlaw action as acceptable, and even glorifies it. Witness cop shows: How many of them get to the end without major violence? As if order could only be achieved by wiping people out.
Look at the cultures and countries with low rates of violence: I'd bet they value debate, negociation, and consistant norms of orderly behaviour. The yakuza in Japan belong to neighbourhood associations.
