Also consider that in your post you refer to "god's rule"? Which god? If you refer to the god of the Bible you are referring to the Judaeo-Christian YHWY and Jesus. The social acceptance that Murder is wrong pre-dates these religions and is found in other religions.
Man has attempted to control the masses since the beginning. Religion was the tool first used. This was followed by government, which first cited religion as the basis for these prohibitions.
Morality is subjective and not objective. Morality is not a constant. Morality is what a majority of people in a society are willing to tolerate.