The “prey” refers to the captives and livestock: the “spoil” to the ornaments and other effects.

And they took all the spoil. It was a sign both of their disinterestedness and modesty that they brought the spoils, which they had taken …

And they took all the spoil
In their cities and castles; and all the prey, both of men and of beasts; all sorts of c…

The Israelites killed the Kings of Midian. They killed Balaam. God's overruling providence brought him there, and their just vengeance found him. I…