Num 35:33
The Israelites were not to desecrate their land by sparing the murderer; as blood, i.e., bloodshed or murder, desecrated the land, and there was no expiation (יכפּר) to the land for the blood that was shed in it, except through the blood of the man who had shed it, i.e., through the execution of the murderer, by which justice would be satisfied.