20 This is what we will do to them: We will let them live, so that God’s wrath will not fall on us for breaking the oath we swore to them.”

Read Joshua 9:20 Using Other Translations

This we will do to them; we will even let them live, lest wrath be upon us, because of the oath which we sware unto them.
This we will do to them: let them live, lest wrath be upon us, because of the oath that we swore to them."
This is what we must do. We must let them live, for divine anger would come upon us if we broke our oath.