Judges 9:56

Thus God rendered the wickedness of Abimelech, which he did unto his father, in slaying his seventy brethren:

Abimelech was the son of Gideon's maidservant, and his wickedness against his father was that he had Gideon's sons slain, murdering his seventy brothers in order to make himself ruler of Israel. God brought upon him the judgment due for that wickedness in killing his seventy brothers. He also utterly destroyed Shechem.

God rendered that wickedness back upon Abimelech himself, repaying him for what he had done. It was brought back upon him as just punishment for the murders. His violent rule ended in his death, and the Shechemites who had helped him were punished as well. In this, sin does not go unpunished.

The verse belongs to the account behind Jotham's parable. That parable condemns the men of Shechem for choosing as ruler a man responsible for his brothers' deaths. It teaches that ambition to rule over others disqualifies a person from leadership.