Numbers 35:33

Do not pollute (lit. falsely flatter). Do not bring evil, as Targum [Onkelos] renders, וְלָא תְחַיְּבוּן, “do not bring guilt.”

Nor contaminate ye the land in which you are, because innocent blood which hath not been avenged will overflow the land, and there is no atonement made for the land upon which innocent blood hath been shed, but by the shedding of the blood of him who shed it.

V’LO TACHANIPHU’ (AND YE SHALL NOT POLLUTE) THE LAND. Because He had said at the beginning: And these things shall be for a statute of judgment unto you throughout your generations in all your habitations, meaning that these judgments apply also outside the Land [of Israel], therefore He mentioned additional stringencies applying to the inhabitants of the Land [of Israel], in honor of the Divine Presence which is [especially] present there, and He warned us not to pollute it and not to defile it.
And the meaning of the term chanuphah [mentioned here, which literally means: “flattery,” or “pollution”] is that which is said with reference to the imprecations: Thou shalt carry out much into the field, and shalt gather in little; Thou shalt plant vineyards and dress them, but thou shalt neither drink of the wine;, Verse 39. Thou shalt have olive-trees throughout all thy borders, but thou shalt not anoint thyself with oil;, Verse 40. All thy trees and the fruit of thy land shall the locust make bare,, Verse 42. for all expressions of chanuphah indicate doing the opposite of that which is seen by or appears to the eyes. This is the punishment [which will come] to the Land because of idolatry, bloodshed, and immorality, just as it is said, will not the Land ‘chanoph techenaph’ (become polluted)?; the earth also ‘chanepha’ (is polluted) under the inhabitants thereof; ‘vatachniphi’ (and thou hast polluted) the Land with thy harlotries. (polluted)” because of the sins of its inhabitants, which means, according to Ramban, that it will do the opposite of that which is its nature, for it will be planted and cultivated but not produce fruit. And the meaning of the term “defilement” [used in the next verse — And thou shalt not ‘defile’ the Land] is that the Land will become defiled so that the Glory of G-d will not dwell therein if there is innocent blood [shed] in it which has not been atoned for by the blood of him that shed it. Thus the Rabbis have said in the Sifre: 161. “‘V’lo tachaniphu’ the Land — this is an admonition against flatterers.” For at first [in Verses 31-32] He warned us against taking a bribe from murderers, and then [in Verse 33 here] He warned us against flattering them because of their high position or their power, or the honor of their family, even without taking a bribe, because if we flatter them, we will thereby cause the Land to “betray” its inhabitants [as explained above].

כי אם בדם שפכו. As if the Torah had written כי אם בדם האיש אשר שופך את הדם, ”only blood spilled by the killer of the murdered party does not contaminate the Holy Land but cleanses it from blood spilled innocently.” [this is the way I understand the author. Do not forget that normally spilling blood in the Holy Land contaminates its soil, whereas here the blood of the murderer does the very oppsosite. Ed.]