Yotam, in his parable comparing Avimelekh to an Atad is stating that Avimelekh lacks any of the constructive and useful qualifications to become a good king and useful leader, he has seized the vacant position by force. And though he promises to protect those who had been persuaded to appoint him, in the end he will destroy them and himself with his self-centered recklessness. 

This chapter serves as a severe caution, providing us with a profile of the anti-ruler who must not be empowered even when the people are justifiably desperate for leadership.  The anti-ruler candidate possesses a lust for power that should immediately be regarded with suspicion and alarm.

Courtesy of the Virtual Beit Midrash, Yeshivat Har Etzion