Charles Spurgeon Commentary Judges 8:19

"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." — Judges 8:19 (ASV)

As the Lord liveth, if ye had saved them alive, I would not slay you.

But now it fell to him to be an avenger of blood according to the law of that region, and to put to death those who had killed his brothers.