They gave him seventy [pieces] of silver out of the house of Baal Berith, with which Abimelech hired vain and light fellows, who followed him.
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).