Albert Barnes Commentary Jeremiah 21:12

"O house of David, thus saith Jehovah, Execute justice in the morning, and deliver him that is robbed out of the hand of the oppressor, lest my wrath go forth like fire, and burn so that none can quench it, because of the evil of your doings." — Jeremiah 21:12 (ASV)

Execute judgment - As the administration of justice was performed in ancient times in person, the welfare of the people depended to a great degree upon the personal qualities of the king (see 2 Samuel 15:4). And as “the oppressor” was generally some powerful noble, it was especially the king’s duty to see that the weaker members of the community were not wronged.