Genesis 4:9 - Ellicott's Commentary On The Whole Bible

And the Lord said unto Cain, Where is Abel thy 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 of which see 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 subtilty, 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 came not, and the question was asked, Where is Abel? the voice of God repeated it in his own heart, Where is Abel, thy 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?”