פּירוש אויף בראשית 4:9

איה הבל אחיך WHERE IS ABEL THY BROTHER — thus entering into a friendly conversation with him: perhaps he might repent and say, “I have killed him, and sinned against You” (Genesis Rabbah 19:11).

“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.

אי הבל אחיך, “in which place did you bury him?” G’d phrased this as a question only in order to give Kayin a chance to begin the process of repentance, not because He was not perfectly aware where Kayin had buried Hevel. G’d did not want to be forced to kill someone already guilty of the death penalty.