Judges 8:19

And he said, they were my brethren, even the sons of my
mother
His brethren by his mother's side, but not by his father's side; or the phrase

the sons of my mother is added,
to show that he did not mean brethren in a large sense, as all the Israelites were, but in a strict sense, being so nearly related as his mother's children:

as the Lord liveth, if ye had saved them alive, I would not slay you;
for not being Canaanites, he was not obliged by the law of God to put them to death, and by the law of nations, as they had surrendered themselves, and were made prisoners of war, they ought to have been saved; but as they appeared to be murderers, and had slain the Israelites in cold blood, they deserved to die; and the persons they had slain being Gideon's brethren, he was the avenger of blood, and it became him to put them to death.