Read Numbers 35:33 Using Other Translations
So ye shall not pollute the land wherein ye are: for blood it defileth the land: and the land cannot be cleansed of the blood that is shed therein, but by the blood of him that shed it.
You shall not pollute the land in which you live, for blood pollutes the land, and no atonement can be made for the land for the blood that is shed in it, except by the blood of the one who shed it.
This will ensure that the land where you live will not be polluted, for murder pollutes the land. And no sacrifice except the execution of the murderer can purify the land from murder.