He went to his father's house at Ophrah, and killed his brothers the sons of Jerubbaal, being seventy persons, on one stone: but Jotham the youngest son of Jerubbaal was left; for he hid himself.
In other words, he's an outsider. We ought to be serving Hamor and his family, let them be the kings. And he said to God the people were under my rule, because I can really do a good job here, you know. Who's Abimelech? And so he said if you would just commit yourself to me I would remove Abimelech.