John Calvin Commentary Numbers 31:19

"And encamp ye without the camp seven days: whosoever hath killed any person, and whosoever hath touched any slain, purify yourselves on the third day and on the seventh day, ye and your captives." — Numbers 31:19 (ASV)

And do you abide without the camp seven days. We have seen elsewhere,²⁰⁹ that if anyone had touched a dead body, he was considered unclean. Moses, by now extending the ceremony of expiation to lawful homicide, suggests how carefully we should abstain from shedding human blood.

It was required of the Israelites that they should strenuously advance through the midst of carnage. But, since it is contrary to the order of nature for humans to be killed by humans—as if they were raging against their own flesh and blood—God willed that some vestiges of humanity be preserved even in just punishments, to put a restraint upon all cruelty in principle.

And it is not without reason that Scripture, even in commending heroic bravery, uses this form of expression, that they have polluted their hands with blood, who have killed any of their enemies; that is, so that we may abhor all acts of homicide as repugnant to the preservation of the human race.

Although, therefore, the Israelites had killed the Midianites not only justly but by God’s command, still, so that they would not accustom themselves to the indiscriminate shedding of blood, they were commanded to purify themselves on the third and the seventh day before they returned to the camp, so that their pollution would not infect the people.

The reason for purifying the booty was different, namely, because the uncleanness of their vessels indicated how detestable this people was, whose very utensils, until they were purified by fire or water, defiled everyone by the mere touch.

However, so that the soldiers would not refuse to obey or comply unwillingly, Eleazar reminds them that nothing more was required of them than the observance of an old ordinance. And undoubtedly, Moses intentionally entrusted the office of teaching to his nephew, because the interpretation of the law was from then on to be sought from the mouth of the priest.

²⁰⁹ See ante, on on Numbers 19:11, , vol. 2, p. 42..