Killing a kafir isn't murder. If the people involved are of equal class it's a murder. If the crime was murder then the accused will be executed under sharia. There was no compensation for murder. There is compensation as one would lose property for killing a kafir.
From an Islamic site:
In the above narrations, we clearly see that Allah Almighty used to punish to death those who commit intentional murders among the People of Israel. This law is still effective in Islam, and killing the murderer is still a valid law in Islam, but there is however another alternative for punishment, and that is accepting by choice the blood money.
If the relatives of the slain person wish to have an open and forgiving heart, then they can forgive the murderer under the condition that he compensate them with money for their slain son or relative. The demand should be reasonable.
If the relatives do not wish to forgive the murderer, then he is to get executed by the Islamic ruling authority.
http://www.answering-christianity.com/laws_of_murder.htm
Which is exactly what I said.
In most slave states prior to our Civil War, killing of a slave without just cause was murder, with the death penalty. It even actually happened. Same was true under Jim Crow. There was, by law, no difference between a white man killing a black man and a black man killing a white man.
However, in practice the consequences were usually, though not always, dramatically different.
You are conflating the fact that in Muslim lands it is unlikely a Muslim will be prosecuted for killing a Christian with the assumption that it is legal for him to do so. AFAIK that is not the case anywhere.
Two different things.