en_tn_condensed/jdg/09/20.md

But if not

"But if you did not act with honesty and integrity with Jerub Baal and his house"

let fire come out from Abimelek and burn up the men of Shechem

Jotham is speaking a curse. He speaks of Abimelek destroying the people of Shechem as if he would burn them with fire. Gideon's son, Abimelech, killed all of his brothers except Jotham in order to become king of Shechem. Jotham cursed Abimelech for having murdered Gideon's other sons.