That is somewhat the point I want to make. There is a law everywhere in this nation against killing, and yet murder rates are all over the place in every city. It seems to me that a law against killing, despite having much the same consequences, does not stifle murder. Rather, I don't want to kill you because I have food, and water, and all the things I could ever hope for (chiefly, a computer with Internet access.) If, however, it was a life and death sort of thing, and you had food and water, or the thing which could get me food and water, that being money, I would kill you if it meant my own survival. Violent crime, it appears to me, is spawned from necessity, and there is a lot of that going on even in this nation, but violent crime is even more prevalent in places where one's survival is threatened.
I get that, and I agree that the reason most people don't kill has nothing to do with the law but you're overlooking a few very important points. First, that we have a mechanism in place to prevent people from killing again if we catch them. Second, that that mechanism is overseen by everyone who lives under that system, and each of those people has a voice in changing that system. I don't mean to be pollyannaish about government, but we truly do have safeguards in place in many parts of the world. The legal disincentive regarding killing prevents many forms of organized murder, for the simple reason that someone might fink you out. Murder is something you pretty much have to do alone or with very close and trusted conspirators. Even that is a huge gamble.
Finally, here's another correlation that I won't pretend is the whole story. Where (and when) the likelihood of being caught and imprisoned is higher, the murder rate is lower. People do, at least sometimes, consider consequences before engaging in violence.