John Gill Commentary Judges 9:20

"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." — Judges 9:20 (ASV)

But if not
If it appeared that they had not acted uprightly and sincerely in this matter:

let fire come out of Abimelech, and devour the men of Shechem, and the
house of Millo ;
let wrath, rage, and fury, break out from Abimelech like fire, and issue in the destruction of those that made him king, both those of Shechem and of Millo:

and let fire come out from the men of Shechem, and from the house of
Millo, and devour Abimelech ;
let them be incensed against Abimelech, and seek his ruin, and procure it: the sense is, that he wishes that strife, contention, and quarrels, might arise among them, and they mutually destroy each other; the words are imprecative of evil upon them both, and which had its exact fulfilment.