John Gill Commentary Numbers 35:34

"And thou shalt not defile the land which ye inhabit, in the midst of which I dwell: for I, Jehovah, dwell in the midst of the children of Israel." — Numbers 35:34 (ASV)

Defile not therefore the land which you shall inherit
By the commission of such atrocious crimes, or suffering them to go unpunished, or by taking a compensation for the life of the guilty person:

in which I dwell ;
which is added to strengthen the exhortation, and as giving a reason why care should be taken not to pollute it, because the Holy God dwells there; as he did in the tabernacle erected for him, and in such a peculiar manner as he did not in other lands:

for I the Lord dwell among the children of Israel ;
he now dwelt among them as their God, and their King; his tent or tabernacle being pitched in the midst of the camps of Israel; and so he would continue to dwell among them when they were come to the land of Canaan, so long as they observed his laws, statutes, and ordinances; and therefore it behoved them to be careful that they did not pollute themselves and their land, and cause him to depart from them.