Bede

He answered: I do not know. Am I my brother's keeper? A foolish and equally arrogant answer; foolish, since he thought he could deceive Him who knew his sinful conscience and accused him of the crime of fratricide even before it was committed, as the knower of secrets; arrogant, since he denied being his brother's keeper in the manner of an obstinate servant, who by manifestation, if any danger threatened, should have taken care of his younger brother, and as the elder should have protected him from impending adversities.

Commentary on Genesis (Hexaemeron) ↗