So ye shall not pollute the land wherein ye are: for blood it defileth the land: and the land cannot be {n} cleansed of the blood that is shed therein, but by the blood of him that shed it.

(n) So God is mindful of the blood wrongfully shed, that he makes his dumb creatures demand vengeance of it.

King James Translators' Notes

the land cannot...: Heb. there can be no expiation for the land

Matthew Henry's Concise Commentary

35:9-34 To show plainly the abhorrence of murder, and to provide the more effectually for the punishment of the murderer, the nearest relation of the deceased, under the title of avenger of blood, (or the redeemer of blood,) in notorious cases, might pursue, and execute vengeance. A distinction is made, not between sudden anger and malice aforethought, both which are the crime of murder; but between intentionally striking a man with any weapon likely to cause death, and an unintentional blow. In the latter case alone, the city of refuge afforded protection. Murder in all its forms, and under all disguises, pollutes a land. Alas! that so many murders, under the name of duels, prize-fights, &c. should pass unpunished. There were six cities of refuge; one or other might be reached in less than a day's journey from any part of the land. To these, man-slayers might flee for refuge, and be safe, till they had a fair trial. If acquitted from the charge, they were protected from the avenger of blood; yet they must continue within the bounds of the city till the death of the high priest. Thus we are reminded that the death of the great High Priest is the only means whereby sins are pardoned, and sinners set at liberty. These cities are plainly alluded to, both in the Old and New Testament, we cannot doubt the typical character of their appointment. Turn ye to the strong hold, ye prisoners of hope, saith the voice of mercy, Zec 9:12, alluding to the city of refuge. St. Paul describes the strong consolation of fleeing for refuge to the hope set before us, in a passage always applied to the gracious appointment of the cities of refuge, Heb 6:18. The rich mercies of salvation, through Christ, prefigured by these cities, demand our regard. 1. Did the ancient city rear its towers of safety on high? See Christ raised up on the cross; and is he not exalted at the right hand of his Father, to be a Prince and a Saviour, to give repentance and remission of sins? 2. Does not the highway of salvation, resemble the smooth and plain path to the city of refuge? Survey the path that leads to the Redeemer. Is there any stumbling-block to be found therein, except that which an evil heart of unbelief supplies for its own fall? 3. Waymarks were set up pointing to the city. And is it not the office of the ministers of the gospel to direct sinners to Him? 4. The gate of the city stood open night and day. Has not Christ declared, Him that cometh unto me I will in nowise cast out? 5. The city of refuge afforded support to every one who entered its walls. Those who have reached the refuge, may live by faith on Him whose flesh is meat indeed, and whose blood is drink indeed. 6. The city was a refuge for all. In the gospel there is no respect of persons. That soul lives not which deserves not Divine wrath; that soul lives not which may not in simple faith hope for salvation and life eternal, through the Son of God.