It is not clear why this verse mentions Beth Millo along with Shechem, especially since Judges 9:2–3, Judges 9:7, and Judges 9:18 speak only of the citizens of Shechem. Beth-millo
may be another way of referring to Shechem’s leaders, given how the phrase stands in apposition to all the leaders of Shechem.
It is best interpreted as another designation for the nobility, those who had authority to participate in Abimelech’s installation as king, perhaps the official administrators of the city.
1
6 And all the leaders of Shechem came together, and all Beth-millo, and they went and made Abimelech king, by the oak of the pillar at Shechem.