Jamieson, Fausset and Brown on Numbers 31:19-24

19-24. abide without the camp seven days: whosoever hath killed any person ... purify both yourselves and your captives--Though the Israelites had taken the field in obedience to the command of God, they had become defiled by contact with the dead. A process of purification was to be undergone, as the law required (Le 15:13; Nu 19:9-12), and this purifying ceremony was extended to dress, houses, tents, to everything on which a dead body had lain, which had been touched by the blood-stained hands of the Israelitish warriors, or which had been the property of idolaters. This became a standing ordinance in all time coming (Le 6:28; 11:33; 15:12).