For being not Canaanites he was not obliged to kill them; but they having killed his brethren, and that in cool blood, he was by law the avenger of their blood.
Related Commentaries of Judges 8:19
Judges 8:19
19 And he said, They were my brethren, even the sons of my mother: as the LORD liveth, if ye had saved them alive, I would not slay you.