Albert Barnes Commentary Judges 9:5

"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: but Jotham the youngest son of Jerubbaal was left; for he hid himself." — Judges 9:5 (ASV)

Such wholesale slaughters have always been common in Eastern monarchies, and are among the fruits of polygamy.