Adam Clarke on Judges 9:25

The men of Shechem set liers in wait - It pleased God to punish this bad man by the very persons who had contributed to his iniquitous elevation. So God often makes the instruments of men's sins the means of their punishment. It is likely that although Abimelech had his chief residence at Shechem, yet he frequently went to Ophrah, the city of his father; his claim to which there was none to oppose, as he had slain all his brethren. It was probably in his passage between those two places that the Shechemites had posted cut-throats, in order to assassinate him; as such men had no moral principle, they robbed and plundered all who came that way.