‘ That the violence done to the seventy sons of Jerubbaal might come, and that their blood might be laid, on Abimelech their brother, who slew them, and on the men of Shechem who strengthened his hands to kill his brothers.'
The reason the evil spirit's activity was allowed and even encouraged was so that vengeance might be gained on Abimelech in accordance with the curse of Jotham and of Yahweh.
“Seventy sons”. A round number indicating all the slain and stressing their acceptance with Yahweh (seven intensified). ‘Jerubbaal' is now used constantly to indicate that Abimelech was mixed up with and followed Baal. All were guilty, both the murderers and those who encouraged it.