Jamieson, Fausset and Brown on Judges 8:20

20. he said unto Jether his first-born, Up, and slay them--The nearest of kin was the blood-avenger; but a magistrate might order any one to do the work of the executioner; and the person selected was always of a rank equal or proportioned to that of the party doomed to suffer (1Ki 2:29). Gideon intended, then, by the order to Jether, to put an honor on his son, by employing him to slay two enemies of his country; and on the youth declining, he performed the bloody deed himself.