What does Judges 9:23 mean?

It didn't take long for Jotham's prophetic curse to be fulfilled (Judges 9:19–20). He had declared that because the leaders of Shechem acted without integrity, fire would come out from both Abimelech and the leaders of Shechem to devour each other. This imagery ties into Jotham's fable, which pictured trees choosing the worthless bramble to be king (Judges 9:8–15), despite it having nothing to offer but disaster.

The curse works itself out through the influence of an evil spirit sent by God. Not all references to spirits "between" persons implies a literal demonic presence. That can be the case, but God's sending of an "evil spirit between" two sides can refer to disagreement, division, hatred, and strife. This is not the only time God uses such methods. He does something similar with King Saul (1 Samuel 16:14). He even allows Jesus to be tempted by the Devil in the wilderness to prove His blamelessness (Matthew 4:1–11). Later, Paul urges the Corinthians to turn an unrepentant sinner in their church over to Satan, so that sinner can be led to repentance and salvation (1 Corinthians 5:4–5). Whether the spirit that plagued Abimelech and Shechem is a being, or an attitude, God has authority over it. His sovereignty includes Satan and demons, and He can make use of even those beings to fulfill His will on earth.

In this case, God's will is to hold Abimelech and the leaders of Shechem responsible for their heinous acts (Judges 9:24). That includes the murder of Gideon's seventy sons (Judges 9:1–6). Judgment will come as the wicked persons turn on each other. After three years, tensions lead to a break between Abimelech and the leaders who put him in that position over Shechem. In some form or another, the leaders of Shechem betray their appointed ruler. They revolt against his control and betray him.