24 God did this in order that the crime against Jerub-Baal’s seventy sons, the shedding of their blood, might be avenged on their brother Abimelek and on the citizens of Shechem, who had helped him murder his brothers.

Read Judges 9:24 Using Other Translations

That the cruelty done to the threescore and ten sons of Jerubbaal might come, and their blood be laid upon Abimelech their brother, which slew them; and upon the men of Shechem, which aided him in the killing of his brethren.
that the violence done to the seventy sons of Jerubbaal might come, and their blood be laid on Abimelech their brother, who killed them, and on the men of Shechem, who strengthened his hands to kill his brothers.
God was punishing Abimelech for murdering Gideon’s seventy sons, and the citizens of Shechem for supporting him in this treachery of murdering his brothers.