Jdg 9:56 Thus God repaid the wickedness of Abimelech, which he had done to his father by killing his seventy brothers.
Jdg 9:57 And all the evil of the men of Shechem God returned on their own heads, and on them came the curse of Jotham the son of Jerubbaal.

The story of Abimelech is an interesting case of a divine curse being fulfilled. Abimelech was son of Gideon via a servant girl. He had his 70 brothers killed in order for him to remain as the sole claimant as tribal leader. One of the 70 sons escaped, cursing Abimelech as he left. This curse seems to have been honored by God, although the curse does not invoke God’s name.

God, after three years, sends a spirit to drive a wedge between Shechem (the city that appointed Abimelech as ruler) and Abimelech:

Jdg 9:23 And God sent an evil spirit between Abimelech and the leaders of Shechem, and the leaders of Shechem dealt treacherously with Abimelech,
Jdg 9:24 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.

Shechem soon turns to new leaders who challenge Abimelech. Abimelech crushes his new rivals. He then burns the leaders of Shechem alive (fulfilling the curse). Abimelech then turns against the city of Thebez, who had apparently joined the Shechem rebellion. It is here he is struck by a falling millstone and dies. This death is attributed to God. God repaid the wickedness of Abimelech.

The story reads as if God is the cause of the flow of the narrative, although there seems to be randomness and coincidence embedded throughout. God sends an “evil” spirit (probably better understood as a mischievous spirit). This spirit’s goal is to drive a wedge between the guilty people and the guilty Abimelech such that they turn their power against each other. God is pitting his enemies against his enemies.

This spirit causes a chain of events which kills not only the people of Shechem but also Abimelech. It could be that God guided the millstone which killed Abemelech, but any circumstance of death for Abimelech would likely be attributed to God repaying Abemelech. God is the reason Abimelech is involved in this civil war in the first place.

God seems to have had vested interest in Shechem’s rebellion failing, such that they receive punishment. God also seems to have vested interest in Abemelech dying in an extended campaign. Fire destroyed Shechem (per the curse) but not Abemelech (against the curse). The fire could be meant figuratively. The curse was that the relationship between Abemelech and Shechem would prove toxic. The evil spirit is explicitly attributed to causing this failed relationship, and no more. It is unclear the extent that God intervened other than this failed relationship, and surely the curse could have been fulfilled in diverse ways (e.g. Abemelech could have died in open combat while his troops defeated Shechem). The use of a spirit as an impetus (after 3 years of non-intervention) implies God is not controlling all things but intervenes where necessary to further the events of history.