2 The imperative is followed by its cognate accusative to stress this vengeance. The Midianites had attempted to destroy Israel with their corrupt pagan practices, and now will be judged. The accounts indicate that the effort by Midian was calculated and evil.
3 The war was commanded by the Lord and was to be divine vengeance on the Midianites. So it was holy war. No Israelites then could take spoils in this—it was not a time for plunder and aggrandizement. It was part of the judgment of God upon those who would destroy or pervert his plan and his people.
4 This would be the last major enterprise that Moses would have to undertake. He would soon die and “be gathered to his people” as Aaron was.

In other versions

“Avenge the people of Israel on the Midianites. Afterward you shall be gathered to your people.”

“Take vengeance on the Midianites for the Israelites. After that, you will be gathered to your people.”

“Execute vengeance for the Israelites against the Midianites. After that, you will be gathered to your people.”

“On behalf of the people of Israel, take revenge on the Midianites for leading them into idolatry. After that, you will die and join your ancestors.”

“Seek vengeance for the Israelites on the Midianites; afterward you will be gathered to your people.”

Avenge the children of Israel of the Midianites: afterward shalt thou be gathered unto thy people.

Public Domain