He said unto Jether his first-born - By the ancient laws of war, prisoners taken in war might be either slain, sold, or kept for slaves. To put a captive enemy to death no executioner was required. Gideon slays Zebah and Zalmunna with his own hand. So Samuel is said to have hewn Agag in pieces, 1 Samuel 15:33. Benaiah slew Joab, 1 Kings 2:25. Saul orders his guards to slay the priests who had contributed to the escape of David, 1 Samuel 22:17; and David caused one of his attendants to slay the Amalekite who pretended to have slain Saul, 2 Samuel 1:15.
It was Gideon‘s place to act the part of the “avenger of blood” Numbers 35:12; Deuteronomy 19:6. The fierce manners of the age break out in the slaying of the captives (compare 1 Samuel 15:32-33), and in Gideon‘s attempt to initiate his youthful son Jether in the stern work of slaying his country‘s enemies.