John Wesley on Joshua 9:15

To let them live - That is, they should not destroy them. That this league was lawful and obliging, appears, Because Joshua and all the princes, upon the review concluded it so to be, and spared them accordingly. Because God punished the violation of it long after, 2Sam 21:1. Because God is said to have hardened the hearts of all other cities, not to seek peace with Israel, that so he might utterly destroy them, Jos 11:19,20, which seems to imply that their utter destruction did not necessarily come upon them by virtue of any peremptory command of God, but by their own obstinate hardness, whereby they refused to make peace with the Israelites.