Genesis 4:9

Where is Habel thy brother? - The interrogatory here reminds us of the question put to the hiding Adam, "Where art thou?" It is calculated to strike the conscience. The reply is different from that of Adam. The sin has now advanced from hasty, incautious yielding to the tempter, to reiterated and deliberate disobedience. Such a sinner must take different ground. Cain, therefore, attempts to parry the question, apparently on the vain supposition that no eye, not even that of the All-seeing, was present to witness the deed. "I know not." In the madness of his confusion he goes further. He disputes the right of the Almighty to make the demand. "Am I my brother's keeper?" There is, as usual, an atom of truth mingled with the amazing falsehood of this surly response. No man is the absolute keeper of his brother, so as to be responsible for his safety when he is not present. This is what Cain means to insinuate. But every man is his brother's keeper so far that he is not himself to lay the hand of violence on him, nor suffer another to do so if he can hinder it. This sort of keeping the Almighty has a right to demand of every one - the first part of it on the ground of mere justice, the second on that of love. But Cain's reply betrays a desperate resort to falsehood, a total estrangement of feeling, a quenching of brotherly love, a predominence of that selfishness which freezes affection and kindles hatred. This is the way of Cain Jde 1:11.

4:9 And the Lord said unto Cain, Where is Abel thy brother? — God knew him to be guilty; yet he asks him, that he might draw from him a confession of his crime; for those who would be justified before God, must accuse themselves.

And he said, I know not — Thus in Cain the devil was both a murderer, and a liar from the beginning. Am I my Brother's keeper? - Sure he is old enough to take care of himself, nor did I ever take charge of him. Art not thou his keeper? If he be missing, on thee be the blame, and not on me, who never undertook to keep him.