Charles Ellicott Commentary Numbers 31:1

"And Jehovah spake unto Moses, saying," — Numbers 31:1 (ASV)

Avenge the children of Israel of the Midianites. —The time had now come for the fulfillment of the command which had already been given , after which Moses was to be gathered to his people, as it had been revealed to him (Numbers 27:13). After Balaam had been dismissed by Balak, he appears to have gone, not to the Moabites, but to the Midianites; and it was in consequence of the counsel which he gave to the Midianites (Numbers 5:16) that the Israelites were led into the idolatrous and lascivious worship of Baal Peor. It is also possible that the Midianites, as the descendants of Abraham, may have possessed clearer light and greater privileges than the Moabites.

They may have had many men as enlightened as Jethro among them; consequently, they may have incurred greater guilt and made themselves liable to the more severe punishment of those who, after they have known the way of righteousness, turn from the holy commandment delivered unto them (2 Peter 2:21).

But whether satisfactory reasons can or cannot be assigned for why a more exemplary judgment was inflicted on the Midianites than on the Moabites (who were not left unpunished), only those can maintain that the destruction of the Midianites is inconsistent with the justice or goodness of God who deny that He has absolute control over the destinies of all the creatures of His hands.

They must also deny that when He is pleased to recall the life which He has bestowed, it is for Him to determine which agents or instruments are best to employ.