Numbers 35:33

33.So ye shall not pollute the land. In this concluding sentence, He again reminds them that, unless they should exercise severe justice against murderers, they would be guilty of sin against God; because the land stained with human blood is polluted, and lying under His curse, until expiation has been made. Again, since God dwells in the land of Canaan, having chosen His abode among the children of Israel, his sanctity is also profaned. The sum is, that, in every respect, care should be taken lest the land, which is sacred to God, should be contaminated by bloodshed.