John Gill 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 you shall not pollute the land where you are
The land of Canaan, as it had been by the old inhabitants of it, by idolatry, adultery, and murder:

for blood it defiles the land :
the shedding of innocent blood defiles a nation, and the inhabitants of it, brings guilt on it, and subjects to punishment:

and the land cannot be cleansed of the blood that is shed in it, but
by the blood of him that shed it ;
or "there can be no expiation" ^F2 , or "atonement made" for it in any other way; the blood of the murderer is required at his hands, and nothing short of it will satisfy law and justice, see (Genesis 9:6) .

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FOOTNOTES:

• F2: (rpky al) "non posset expiatio", Junius & Tremellius, Piscator; to the same sense Pagninus, Montanus, Tigurine version.