Charles Ellicott Commentary Judges 8:20

"And he said unto Jether his first-born, Up, and slay them. But the youth drew not his sword; for he feared, because he was yet a youth." — Judges 8:20 (ASV)

And he said to Jether. — By the jus talionis, as well as by every other consideration of that time, Gideon, as the last survivor of all his kingly brothers, would hold himself justified in putting his captives to death. Jether also would inherit the duties of goel (Numbers 35:12; 2 Samuel 2:22, and other passages), and Gideon desired both to train the boy to fearlessness against the enemies of Israel (Joshua 10:24–25), to give him prestige, and to add to the disgrace of the Midianite kings.

Again. Gideon must only be judged by the standard and the customs of his own day (Compare 1 Samuel 15:33, Samuel and Agag; 2 Samuel 1:15, David and the Amalekite). The name Jether is another form of Jethro and means “pre-eminence.”