Joel 2:13 - Ellicott's Commentary On The Whole Bible

Repenteth him of the evil — i.e., in the sense that of His own will He would not the death of a sinner. The judgments of God, like His mercies, are conditional. As the “Lord repented (i.e., grieved) that He had made Saul king over Israel,” and revoked the appointment, so now He repenteth Him of the evil which will fall on His people if impenitent. If they will repent, it may be He will do it not.