John Gill Commentary Judges 9:23

"And God sent an evil spirit between Abimelech and the men of Shechem; and the men of Shechem dealt treacherously with Abimelech:" — Judges 9:23 (ASV)

Then God sent an evil spirit between Abimelech and the men of
Shechem
Permitted, yes, gave a commission to Satan, the evil spirit, to go among them, who stirred up suspicions, jealousies, hatred, and ill will to one another, and sowed the seeds of discord and contention among them; or God gave them up to their own hearts' lusts, to think ill of one another, grow jealous, and meditate revenge.

and the men of Shechem dealt treacherously with Abimelech ;
did not openly declare their minds, but secretly conspired against him, and privately consulted ways to find means to get rid of him, and shake off his government.