I don’t speak Hebrew and have only read Bamidbar 35 in English translations. The JPS Tanakh renders verse 33 as ‘You shall not pollute the land in which you live; for blood pollutes the land, and the land can have no expiation for blood that is shed on it, except by the blood of him who shed it.’ To my English ears this sounds as if Hashem is contradicting His own law concerning accidental killers. The English phrase ‘blood that is shed’ implies nothing about the motives of the blood-shedder. The translation as a whole implies, in English, that a man-slayer ought to be killed. Is this a case of misleading translation?
You should note that the previous verses give the halacha of both the intentional murderer and the accidental murderer. The pesukim also give the halacha that one cannot attempt to pardon a murderer (intentional or accidental) from the appropriate punishment. Ms'ei 35:31-32
31 You shall not accept ransom for the life of a murderer, who is guilty of death, for he shall be put to death.
32 You shall not accept ransom for one who has fled to his city of refuge, to allow him to return to live in the Land, before the kohen has died.
Rav Hirsch translates pasuk 33 as
And ye shall not make the land in which ye are into a hypocrite, for blood, it maketh the land into a hypocrite, and for the land there is no atonement for the blood that is shed in it except by the blood of him that shed it.
Thus not punishing either type of killing appropriately will pollute or corrupt the land. Such an action in either case causes the land to require the redemption of the blood that was spilled.
Rav Hirsch explains:
חנף, as is definitely clear in its use in Job and Proverbs means: to play the hypocrite, to put on an air of piety; to present an appearance externally which by no means corresponds to the inner reality. So that he who makes a man into a hypocrite takes away the inner kernel leaving the outer appearance unchanged.
Now it says: ולא תחניפו את הארץ, if you tolerate intentional murder and careless manslaughter, then you make the land אשר אתם בו in which you are (not merely אשר אתם יושבים בה as in V34), in which your earthly existence has its roots, which is to see it noblest product in you, you make the land into a "hypocrite". It deceives the expectation which otherwise you are justified in entertaining from it, it keeps back the blessing that should flow out from it, "כִּ֣י הַדָּ֔ם ה֥וּא יַֽחֲנִ֖יף אֶת־הָאָ֑רֶץ", for blood, human blood is the noblest fruit that the land matures, and innocent blood spilt makes the land into a "hypocrite".
this חנופה of the land is only purged when the innocent blood which has been spilt, and the human being whose existence down here has been brought to an end thereby, finds a champion, amongst the human society which goes on living after it, to take up its cause, and the murderer atones for his existence down here which was forfeited by the murderer by being killed by that champion of the murder. For by shedding the blood of his brother-man, his own blood has lost its justification, he himself had forfeited the right to go on living; and tolerating the further continuing living on of one who has deliberately and with foresight murdered a fellow-man, is an insult to and derision of the higher dignity of the conception of Man being near God, is a breach of the contract under which God gave the earth to Man, under which God had given the land to Israel.
The case of accidental death has to be explained specifically because the person who negligently causes someone else's death cannot be allowed to go completely free. His actions did cause someone else's death but while it was without intent, he should have taken care to ensure that the circumstance did not take place. Thus the blood of the dead person must be atoned for and he must show that there is an element of guilt. That is why both verses 31 and 32 are written.
What you're saying is theoretically true, but there's no contradiction. It's not so much a mistranslation as an incomplete one. Direct translation minus the explanation can be misleading.
First off, remember that even a negligent murderer can be killed, by the "blood redeemers" i.e. members of the victims family, as a result of his crime. He is given the ability/obligation to flee to a city of refuge to save his life; but if he leaves there, he is a dead man!
Now here's the explanation of the Chizkuni, one of the earlier commentators, explaining the verse (translation is from Sefaria, link's here 1
ולא תחניפו את הארץ
, “this verse is to be understood at face value, i.e. “you shall not flatter the land;” this is a warning for wealthy people who had become guilty of the death penalty not to be allowed to plead to a lesser charge, and to make generous financial restitution to the party whom they had harmed or to the family of that person. 'כי הדם הוא הוא יחניף וגו, “for the blood spilled (and not atoned for) would contaminate” the soil of the Holy Land. This is why G-d warns us not to contaminate the land he has given us by allowing bloodshed to go unpunished according to the principle of נפש תחת נפש, “a life for a life,” a principle spelled out already in Genesis 9,6, a principle applicable to all of mankind.
(I bolded the extra words of explanation which he adds, which clarifies it a bit.)
Put this in context with the previous verse which allow for no monetary restitution to take the place of execution (for an intentional murderer) or exile (for a negligent one.)
Allowing a murderer to walk around free, while his victim's blood is in the ground, would contaminate it. Thus we must keep murderers off the land- either by killing them or locking them in cities of refuge, depending on the crime.
But it doesn't mean that we are supposed to kill negligent murderers. Thus there's no contradiction.