Such wholesale slaughters have always been common in Eastern monarchies, and are among the fruits of polygamy.
Related Commentaries of Judges 9:5
Judges 9:5
5 And he went unto his father's house at Ophrah, and slew his brethren the sons of Jerubbaal, being threescore and ten persons, upon one stone: notwithstanding yet Jotham the youngest son of Jerubbaal was left; for he hid himself.