Judges 8:19

‘ And he said, “They were my brothers, even the sons of my mother. As Yahweh lives, if you had saved them alive, I would not kill you.” '

They were guilty out of their own mouths and had determined their own punishment. They had by their actions forfeited mercy. As they had done, so would be done to them (compare Judges 1:7). This was the principle on which justice was determined. We learn also here why Gideon was such a good choice to lead against the Midianites. Not only was he a man faithful to Yahweh and a leader of men, but he fulfilled his family responsibility to bring to justice the men who had killed his brothers, not in battle but by execution. His deep personal grievance against them would have heightened his determination to bring them to justice.

“The sons of my mother.” His full brothers, sons of his mother. This is not denying that they had the same father.