Jamieson, Fausset and Brown on Judges 9:5

5. went unto ... Ophrah, and slew his brethren i. e., upon one stone--This is the first mention of a barbarous atrocity which has, with appalling frequency, been perpetrated in the despotic countries of the East--that of one son of the deceased monarch usurping the throne and hastening to confirm himself in the possession by the massacre of all the natural or legitimate competitors. Abimelech slew his brethren on one stone, either by dashing them from one rock, or sacrificing them on one stone altar, in revenge for the demolition of Baal's altar by their father. This latter view is the more probable, from the Shechemites (Jud 9:24) aiding in it.

threescore and ten persons--A round number is used, but it is evident that two are wanting to complete that number.