וְלֹֽא־תַחֲנִ֣יפוּ אֶת־הָאָ֗רֶץ אֲשֶׁ֤ר אַתֶּם֙ בָּ֔הּ כִּ֣י הַדָּ֔ם ה֥וּא יַחֲנִ֖יף אֶת־הָאָ֑רֶץ וְלָאָ֣רֶץ לֹֽא־יְכֻפַּ֗ר לַדָּם֙ אֲשֶׁ֣ר שֻׁפַּךְ־בָּ֔הּ כִּי־אִ֖ם בְּדַ֥ם שֹׁפְכֽוֹ׃
Y no contaminaréis la tierra donde estuviereis: porque esta sangre amancillará la tierra: y la tierra no será expiada de la sangre que fué derramada en ella, sino por la sangre del que la derramó.
Kli Yakar
“And God said to Cain, ‘Where is Abel your brother?’” There is great puzzlement regarding Cain — how could he say I do not know? If he erred in thinking that God does not see and does not understand the actions of those below, then why did he bring an offering to God? It seems appropriate to say that Cain certainly understood that the Holy One, Blessed be He, was asking about the killing of his brother, and he responded I do not know, am I my brother’s guardian? Meaning to say, “I did not know there was any sin in the act of killing.” And am I my brother’s guardian is not meant as a question, but rather connects back to I did not know — for he said, “I did not know if I am obligated to protect my brother’s life from being killed by my hand.” This guardianship is in the sense of but guard his life (Job 2:6). Thus Cain argued that he did not know whether he was obligated to protect his brother’s life or not. The Holy One, Blessed be He, replied to him, “How could you not know? This commandment is among those that are self-evident!” This is what is meant by The voice of your brother’s blood cries out to Me from the ground — that reason itself compels one to fulfill this commandment even if it had not been commanded, for without it, people would swallow each other alive. And the land would become desolate without inhabitants (Isaiah 6:11). Therefore, the ground demands justice for the murdered one, because the existence of the earth depends on this — for if there were no restriction on killing one’s brother, it would be impossible for the earth to exist, as its existence would be pointless and it would be desolate without inhabitants. Therefore, the ground shall no longer yield its strength to you. The reason for this is that just as the ground is affected by and seeks judgment for murder, so too the ground itself is obligated to execute judgment on murderers, as it is written (Numbers 35:33), For blood pollutes the land, and the land cannot be cleansed of the blood that is shed in it except by the blood of the one who shed it. And one should contemplate in what way blood pollutes the land.
Sforno
ואני הנני מקים את בריתי, this promise is conditional on man keeping his part of the bargain, bringing murderers to justice. If, however, innocent blood will be shed in the manner it had been before the deluge, this may lead to destruction of the earth (not necessarily by water). This is what is meant in Numbers 35,33 הדם הוא יחניף את הארץ ולארץ לא יכופר לדם אשר שפך בה כי אם בדם שפכו “for blood turns the earth into a hypocrite and there will be no atonement for the land except through the blood of him who has spilled it.”
Chizkuni
ואתה תבער, “So You shall remove, etc;” Rashi explains here that the reference here is that in the event that the murderer is found after the eglah arufah, the heifer that was supposed to atone for the deed, had already been put to death, the people who had killed that beast should not be held responsible for wasting its life, and the murderer will be dealt with judicially. Although Rashi says that he will be executed, seeing that there were neither witnesses nor warning how can that be? [Rashi, of course quoted the Talmud, Sotah folio 47 to that effect. Ed.] If you were to ask that we have learned in the Talmud in tractate Ketuvot, folio 37, that the meaning of the verse in Numbers 35,33: ולארץ לא יכופר לדם אשר שופך בה, “but no expiation can be made for the land for the blood that is shed therein (of innocent people)”, so why was this verse necessary at all? The answer is in order to give us guidance for the scenario described by Rashi, as quoted in the Talmud Sotah 47. Why do we need the request that G-d should remove the innocent blood? It is to teach that all murderers are basically compared to the paragraph dealing with the heifer discussed in our chapter. Just as that heifer’s death is caused by breaking its neck, the neck of all murderers is broken as part of the execution, [Cutting off his neck with a sword. Ed.] On the other hand, that leaves the problem with the sage who holds that the death penalty for murder is strangulation. That death penalty is rated as harsher than the death penalty by cutting off one’s head with the sword. [The reader must remember that whereas the two death penalties by stoning or burning have been spelled out in the Torah, the other two types of death penalties have not been spelled out. Ed.] If not for the verse in the Talmud in tractate Ketuvot folio 37, we would have thought that the sin of murder warrants a more severe penalty that cutting off the head. Our author pursues these details; I have decided that they are not of great interest to either a potential victim or an innocent bystander. Ed.