It’s not the police that keep us from killing each other, or even laws that do so. Check out law and authority sometime if you’re able. It’s very short and worth a read. We don’t kill each other because we don’t enjoy being killed or killing. We’re social creatures, and don’t want to be shunned. Crimes of passion don’t really change based on laws, but the way we organize society may actually be increasing the number of murders, because some people are desperate enough to kill for food or shelter