What does Judges 9:18 mean?

'But today, you’ve turned against my father’s family. You killed his seventy sons on one stone and made Abimelech, the son of his servant woman, king over Shechem just because he’s your relative.' Judges 9:18 - Modern Text Bible

(Jotham accuses them of betraying Gideon's family, murdering his sons, and choosing Abimelech just because of family ties.)