What does Judges 9:7 mean?

To be named king of Shechem, Abimelech committed a series of atrocities. He has killed almost all the other sons of his father Gideon (Judges 8:30). This appears to have been done in something like a ritual execution, one after another (Judges 9:5). This violence missed the youngest brother, though, who successfully hid himself away when Abimelech arrived with his hired goons (Judges 9:1–4). Since Abimelech was eliminating other heirs of Gideon, he may have targeted his nephews—Gideon's grandsons—as well.

Jotham has learned of Abimelech's coronation as king over Shechem. He comes out of hiding to deliver what could be described as a prophetic fable. He picks a spot out of reach up on Mount Gerizim; this overlooks the city from the south. Given the right terrain, a steep hillside can project sound some distance away (Matthew 5:1–2). Jotham calls out his speech in a loud voice, knowing he can be heard without being captured.

Aspects of this passage would be at home in Shakespearean dramas or epic movies. This scene revolves around brothers in three different states. Slain brothers—nearly seventy, at least—lay dead: executed by one of their own, to secure his power and position. High above a town, on a mountain, the only survivor of the massacre delivers a prophetic fable. Below, in the city, the people of Shechem are crowning the murderous brother king.

Jotham warns the leaders of Shechem to listen to him if they want God to listen to them. Apparently, Jotham believes the people of Shechem may yet be redeemed. God can forgive them if they don't give themselves fully to Abimelech. The entire speech which follows is directed to the people; Jotham never bothers to address the murderer of his other brothers. Abimelech's damnation seems inevitable.