Genesis 4:9 - The Popular Commentary by Paul E. Kretzmann

And the Lord said unto Cain, Where is Abel, thy brother? The arraignment of God in this case is like that against Adam and Eve after their transgression. The Lord confronts the murderer with a direct question regarding the whereabouts of his brother Abel, with the intention of working repentance in his heart. And he said, I know not. Am I my brother's keeper? That is the attitude of the hardened sinner, to deny all responsibility, to challenge the Lord with a bold lie: I don't know; am I supposed to be my brother's special keeper and guardian? Sin, willfully committed, always hardens the heart, until all hope of repentance, of a godly sorrow, is futile.