What does Judges 9:20 mean?
Jotham brings his proclamation from the mountaintop (Judges 9:7) to an abrupt end. This is a prophecy of destruction for all below: the people of Shechem and the murderous Abimelech, whom they anointed after paying him to kill Gideon's sons (Judges 9:1–6). This is also described as a curse of vengeance from the sole survivor of Abimelech's massacre.The uninvited speech included a fable about trees, who selected the worthless bramble as their king (Judges 9:8–15). The point of Jotham's tirade is that Shechem has not acted sincerely or honestly. They funded murder, dishonored Gideon, and are anointing someone they know full well is not qualified for the role. In the fable, the bramble promised fire and death if his new subjects were not acting "in good faith."
Knowing Shechem has not acted "in good faith," Jotham prophecies fire—meaning destruction and ruin—will come from Abimelech and devour the leaders of Shechem. It will also consume the well-to-do area inside Shechem known as Beth-millo. As well, they will send death and harm back on Abimelech. This curse predicts that the new king and citizens of Shechem will destroy each other. The story that follows shows how true Jotham's prophecy was (Judges 9:56–57).