Matthew Poole and continuators on Judges 9:20

20 But if not, let fire come out from Abimelech, and devour the men of Shechem, and the house of Millo: and let fire come out from the men of Shechem, and from the house of Millo, and devour Abimelech .

This is not a Prediction, but an Imprecation or Curse, as it is called v. 57. which, being grounded upon just cause, and being the onely way by which Iotham could perform the Duty of the avenger of his Brethrens Blood, which was incumbent upon him; had its effect, as others in like case had, as Ios. 6. 26. compared with 1 King. 16. 34. and 2 King. 2. 24.