The Lord sent a spirit of ill will between Abimelech and the men of Shechem. The writer of Judges explained why the Lord sent this spirit,

…that the crime done to the seventy sons of Jerubbaal [the other name for Gideon, DR], might be settled and their blood be laid on Abimelech their brother, who killed them, and on the men of Shechem, who aided him in the killing of his brothers (Judges 9.24).

This is simply a continuation of incidental references to “seven,” that are found in Judges 9.