Matthew Henry Commentary Judges 8:18-21

"Then said he unto Zebah and Zalmunna, What manner of men were they whom ye slew at Tabor? And they answered, As thou art, so were they; each one resembled the children of a king. And he said, They were my brethren, the sons of my mother: as Jehovah liveth, if ye had saved them alive, I would not slay you. 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. Then Zebah and Zalmunna said, Rise thou, and fall upon us; for as the man is, so is his strength. And Gideon arose, and slew Zebah and Zalmunna, and took the crescents that were on their camels` necks." — Judges 8:18-21 (ASV)

The kings of Midian must be reckoned with. As they confessed themselves guilty of murder, Gideon acted as the avenger of blood, being the next of kin to the persons slain. Little did they think they would hear of this so long afterward; but murder seldom goes unpunished in this life. Sins long forgotten by people must be accounted for to God. What poor consolation in death from the hope of suffering less pain, and of dying with less disgrace than some others! Yet many are more anxious about these matters than about the future judgment and what will follow.