Albert Barnes 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)

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 to 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.