19 "Now here's what you are to do: Pitch tents outside the camp. All who have killed anyone or touched a corpse must stay outside the camp for seven days. Purify yourselves and your captives on the third and seventh days.

Matthew Henry's Commentary on Numbers 31:19

Commentary on Numbers 31:19-24

(Read Numbers 31:19-24)

The Israelites had to purify themselves according to the law, and to abide without the camp seven days, though they had not contracted any moral guilt, the war being just and lawful, and commanded by God. Thus God would preserve in their minds a dread and detestation of shedding blood. The spoil had been used by Midianites, and being now come into the possession of Israelites, it was fit that it should be purified.