Adam Clarke on Judges 9:5

Slew his brethren - His brothers by the father's side, Jdg 8:30. This was a usual way of securing an ill-gotten throne; the person who had no right destroying all those that had right, that he might have no competitors.

Yet Jotham - was left - That is, all the seventy were killed except Jotham, if there were not seventy besides Jotham. All the histories of all the nations of the earth are full of cruelties similar to those of Abimelech: cousins, uncles, brothers, husbands, and fathers have been murdered by their cousins, nephews, brothers, wives, and children, in order that they might have the undisturbed possession of an ill-gotten throne. Europe, Asia, and Africa, can witness all this. Even now, some of these horribly obtained governments exist.