John Calvin Commentary Numbers 35:33

"So ye shall not pollute the land wherein ye are: for blood, it polluteth the land; and no expiation can be made for the land for the blood that is shed therein, but by the blood of him that shed it." — Numbers 35:33 (ASV)

So ye shall not pollute the land. In this concluding sentence, He again reminds them that unless they exercise severe justice against murderers, they would be guilty of sin against God. This is because the land stained with human blood is polluted and lies under His curse until expiation has been made.

Again, since God dwells in the land of Canaan, having chosen His dwelling place among the children of Israel, His sanctity is also profaned.

In summary, care should be taken in every respect so that the land, which is sacred to God, is not contaminated by bloodshed.