Adam Clarke on Judges 8:20

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, 1Sam 15:33. Benaiah slew Joab, 1Kgs 2:25. Saul orders his guards to slay the priests who had contributed to the escape of David, 1Sam 22:17; and David caused one of his attendants to slay the Amalekite who pretended to have slain Saul, 2Sam 1:15.