Abimelech, one of Gideon’s (Jerubbaal) seventy sons enlisted the support of his mother’s brethren and that of the men of Shechem. He then went ahead and slew his seventy brothers, the sons of Jerubbal, but Jotham the youngest escaped and hid himself.
The men of Shechem then made Abimelech king. However Jotham confronted the men of Shechem by telling them a parable on how they have betrayed his father after all he did for them, in killing his sons. He eventually ran away, but not before placing a curse on the men of Shechem and Abimelech.
Three years after Abimelech has been reigning over the men of Shechem, God sent an evil spirit between them, and the men of Shechem began to deal falsely with Abimelech, robbing from him. With Gaal leading the rebelion, they set out to war against Abimelech. Abimelech however defeated Gaal and they were chased out. Abimelech further took the city they moved to and slew the people, burning in the people who went to hid in the stronghold in the city.
It seemed Abimelech was invisible until he went to Thebez. He took the city but the people ran to the strong tower of the city. While he plotted to burn down the tower, a certain (unnamed) woman dropped a millstone on his head. In order to avoid the shame of dying from the hand of a woman, he requested one of his young men slay him.
So, God punished the sin of Abimelech and the betrayalof the men of Shechem, just like Jotham cursed them it came to pass.