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.
Evidently Abimelech felt that Gideon’s other sons were ambitious to be king too, though there is no indication in the text that any of them felt this way. He was perhaps projecting his feelings on them, as is often true of ambitious people. They sometimes become paranoid, as Abimelech did (cf. King Saul).