Charles Ellicott Commentary Genesis 4:9

"And Jehovah said unto Cain, Where is Abel thy brother? And he said, I know not: am I my brother`s keeper?" — Genesis 4:9 (ASV)

And the Lord said to Cain, Where is Abel your brother? — It is the beauty of these early narratives that the dealings of the Deity with mankind are all clothed in an anthropomorphic form, for the reasons for which, see the Note on Genesis 2:7. It seems, then, that Cain at first went away, scarcely conscious of the greatness of his crime. He had asserted his rights, had suppressed the usurpation of his privileges by the younger son, and if he had used force it was his brother’s fault for resisting him.

So Jacob afterwards won the birthright by cunning, and would have paid the same fearful penalty but for timely flight, and rich presents afterwards. But Cain could not quiet his conscience; remorse tracked his footsteps; and when in the household Abel did not come, and the question was asked, Where is Abel? the voice of God repeated it in his own heart, Where is Abel, your brother!—brother still, and offspring of the same womb, even if too prosperous. But the strong-willed man resists. What has he to do with Abel? Is he his brother’s keeper?