Judges

1 Abimelech son of Jerub-Baal went to his mother's brothers in Shechem and said to them and to all his mother's clan, 2 "Ask all the citizens of Shechem, 'Which is better for you: to have all seventy of Jerub-Baal's sons rule over you, or just one man?' Remember, I am your flesh and blood." So it sounds to me like he is doing what our presidents are doing, trying to win over people.
When the brothers repeated all this to the citizens of Shechem, they were inclined to follow Abimelech, for they said, "He is our brother." 4 They gave him seventy shekels of silver from the temple of Baal-Berith, and Abimelech used it to hire reckless adventurers, who became his followers.
And it worked.
He went to his father's home in Ophrah and on one stone murdered his seventy brothers, the sons of Jerub-Baal. But Jotham, the youngest son of Jerub-Baal, escaped by hiding.
So he went and wiped out the descendants of the former king. And I know that is very bad. There are many bad things in this but I am going to talk about the main 2 reasons. It is murder and fear of man. So his fear drove him to murder because the only reason he did this is because they had a right to be king. So they are a threat to his reign. And with that he caused his followers to sin.