Judges – Chapter 9 The Rise and Fall of Avimelech
The curse was that those who rely of improper leaders will eventually be destroyed by those leaders.
Note that Gideon was Yerubaal (because he destroyed the idol) and Yosam is another transliteration of Jotham. Thus "curse" of Yosam was that he declared that the people of Shechem would be punished by being destroyed by the "thornbush" (inappropriate leader) that they backed and would be punished for the murder of Gideon's children.
Note that the thornbush is something that would catch fire and destroy those things that are wrapped in it ("protected" by it). This shows that the burning of the tower and the death of the people of Shechem was "midah keneged middah" (tit for tat).
The Rise and Fall of Avimelech By Rabbi Jack Abramowitz
Avimelech stirred up trouble in the city of Shechem. He suggested that
all 70 of Gidon’s sons would become rulers over the nation and
wouldn’t it be better if there was just one (i.e., him)? The people of
Shechem were from Avimelech’s mother’s side of the family. They gave
Avimelech 70 pieces of silver with which he hired mercenaries who
killed all the other sons of Gidon, except for Yosam, the youngest,
who hid. (Avimelech had them killed at a particular rock, a fact that
will become important later on.) The people of Shechem then crowned
Yosam then called out to the people of Shechem. He used a parable
likening the great leaders Osniel, Devorah and Gidon to an olive tree,
a fig tree and a grape vine, all of whom refused to be monarch over
the nation, but Avimelech is like a thornbush – no fruit, only fuel
for fire and destruction. Since the people of Shechem acted with
malice against Gidon’s family, they would be repaid in kind.
Avimelech ruled for three years, after which time the people of
Shechem rebelled. Gaal son of Eved stirred up opposition against
Avimelech and offered to depose him. Z’vul, a loyal city leader, sent
word to Avimelech warning of the mutiny and asking him to put it down.
Avimelech’s army went forth and battled Gaal’s forces. Avimelech set
fire to the tower of Shechem and killed about a thousand people.
Proceeding to Teibeitz, Avimelech started to lay siege to another
tower, but a woman dropped a millstone on his head, mortally wounding
him. Avimelech asked his armorbearer to kill him so that history would
not record that he was killed by a woman. After Avimelech’s death, his
army dispersed. Both Avimelech and the people of Shechem were repaid
for their treachery against Gidon’s family.
So, remember that Avimelech had his brothers killed at a particular
rock? He was repaid in kind by being killed by a rock. Furthermore, we
see the power of ego. Avimelech was so vain that with his dying breath
he was worried that history would record that he was killed by a
woman. In the end, not only does the Bible record that a woman killed
him, but that he was so worried about his ego, even in death.