Albert Barnes Commentary 1 Kings 11:21

"And when Hadad heard in Egypt that David slept with his fathers, and that Joab the captain of the host was dead, Hadad said to Pharaoh, Let me depart, that I may go to mine own country." — 1 Kings 11:21 (ASV)

That Hadad waited for the death of Joab before requesting leave to return to Idumaea shows what a terrible impression had been made by the severe measures that commander carried out twenty-five or thirty years previously (1 Kings 11:16). The inability of refugees to depart from an eastern court without the king’s permission, and his usual unwillingness to grant it, are illustrated by many passages in the history of Persia.