Numbers 31:8

8. Slew the kings of Midian “Our translators have not exhibited the distinction between the two Hebrew words here employed. Render thus: And the kings of Midian they put to death, besides those that fell in battle; namely, etc. From which it would seem that besides these five, put to death after battle, there were other Midianitish kings who perished fighting; and also that Balaam (chap. xxii-xxiv, notes) did not fall in battle, but was judicially executed.” Canon Cook.

Five kings Vassals of Sihon, the Amorite. Joshua 13:21, note. These, together with Balaam, were reserved for a more formal execution because they were instigators (Numbers 31:16) of the wicked plot to draw Israel into the pollution of idolatry and whoredom.