What does Judges 9:16 mean?

Jotham stands on top of a mountain overlooking the city of Shechem. Below, Jotham's only living brother, Abimelech, is being crowned king. Abimelech brutally murdered the rest of their seventy brothers, sons of Gideon, to become the undisputed king of Shechem (Judges 9:1–6). He would have murdered Jotham, as well, if he had not hidden during the slaughter.

From his perch, Jotham has told a fable about trees looking for a king (Judges 9:6–15). Valuable examples like the olive, fig, and grape refused to become king. They were content with productive and honorable lives. The bramble, however, was willing to take power over others since it had no other value. However, the bramble becomes king of the trees with a promise: to burn them if they have not acted with honesty and sincerity towards him.

Jotham now tells the people of Shechem their choice of Abimelech will turn out well if two things are true. Have they acted in good faith and integrity in making Abimelech their king? And have they honored Gideon—called by his alternate name, Jerubbaal (Judges 6:32)—and his family as his honorable deeds deserved? The implication is that Abimelech is like the bramble: an unsuitable, worthless choice liable to only bring disaster.

As he continues, Jotham clarifies that neither of these things are true. The people of Shechem paid for the slaughter of Gideon's sons, giving money to hire the goons that went with Abimelech. They've chosen Abimelech only because he is not one of Gideon's other sons. Jotham's prophecy will continue to predict that not all will be well.