What does Judges 9:43 mean?
Abimelech is not content with simply defeating the leaders of Shechem in their rebellion (Judges 9:38–41). He wants to punish the entire population for not standing with him against the city's leadership.Following the battle of the day before, the people make their way out into the fields outside the city walls. They seem intent on carrying out their regular duties: caring for crops or other necessary work. There's no explicit mention that any of the regular citizens were directly involved in the rebellion against Abimelech the day before. They are clearly unconcerned about their safety as they leave the city.
When Abimelech hears the people are returning to life as usual, he decides to kill them all. He divides his fighting men into three companies. As he did the day before, he sets an ambush in the hills near the fields (Judges 9:32–34). When the people emerge from of the gates of Shechem, he triggers his plan for a massacre.