The Midianites - The Moabites are not included. It would thus seem that it was the Midianites, and they only, who deliberately set themselves to work the corruption of Israel.

Gathered unto thy people - Where? Not in the grave surely. Moses was gathered with none of them, his burial-place no man ever knew. "But being gathered unto one's people means dying." It does imply dying, but it does not mean this only. The truth is, God considers all those who are dead to men in a state of conscious existence in another world. Therefore he calls himself the God of Abraham, and of Isaac, and of Jacob; now God is not the God of the dead, but of the living; because all Live to Him, whether dead to men or not. Moses therefore was to be gathered to his people - to enter into that republic of Israel which, having died in the faith, fear, and love of God, were now living in a state of conscious blessedness beyond the confines of the grave. See the note on Genesis 25:8, and Genesis 49:33 (note).

Avenge the children of Israel of the Midianites,.... For the injury they had done them, by sending their daughters among them, who enticed them to commit uncleanness with them, and then drew them into the worship of their idols, which brought the wrath of God upon them, and for which 24,000 persons were slain. Now, though the Moabites had a concern in this affair as well as the Midianites, yet they were spared; which some think was for the sake of Lot, from whom they descended; but why not the Midianites for the sake of Abraham, whose offspring they were by Keturah? Jarchi says, they were spared because of Ruth, who was to spring from them; and so she might, and yet vengeance be taken on great numbers of them: but the truer reason seems to be, either because the sin of the Moabites was not yet full, and they were reserved for a later punishment; or rather because they were not the principal actors in the above affair; but the Midianites, who seem to have advised Balak at first to send for Balaam to curse Israel, and who harboured that soothsayer after he had been dismissed by Balak, and to whom he gave his wicked counsel, and which they readily followed, and industriously pursued:
afterward shalt thou be gathered unto thy people; or die, see Numbers 27:13, it being some satisfaction to him to see the good land, as he did from Abarim, and the Israelites avenged on their enemies before his death.