According to the book of Joshua, when Joshua and the Israelites saw that their plan worked and the city was on fire, they fought the people of Ai and won. They were happy because their clever plan helped them win the battle.
Joshua 8:21: And when Joshua and all Israel saw that the ambush had taken the city, and that the smoke of the city ascended, then they turned again, and slew the men of Ai.
The contradiction or conflict in Joshua 8:21 could be seen in the context of violence and warfare. Some might find it contradictory to the broader teachings of compassion and peace because it involves destruction and death. This can seem inconsistent with the message of loving others and promoting peace found elsewhere.