Numbers 31:19

And do ye abide without the camp seven days— The whole army were to stay without the camp seven days; and such of them as had stained their hands in blood, or touched a dead body, were to purify themselves by the water of separation, ch. Numbers 19:9. For, though it was lawful to kill men in a just war; yet, because of the common affinity which subsists between all mankind, and to preserve sentiments of humanity, it was thought fit and decent to oblige all who had shed blood, as well as those who had touched the slain, to undergo a purification, before they were admitted to free conversation and public worship. It appears to have been a very ancient custom among most nations, to appoint certain purifications in all such cases; in order, no doubt, to inspire an uncommon dread and horror of bloodshed. On these occasions, it was usual, particularly, to wash their hands in water, for the purification of the defilement. Thus Homer makes Hector declare himself unfit for performing any offices of divine worship before he was purified:

Ill fits it me, with human gore distain'd, To the pure skies these horrid hands to raise, Or offer heav'n's great Sire polluted praise. POPE, Iliad. 6:
And Virgil makes AEneas say the same thing:
These hands, yet horrid with the stains of war, Refrain their touch unhallow'd, till the day When the pure stream shall wash their guilt away. PITT, AEn. 2:
It was upon this account that the man-slayer, who had involuntarily shed blood, was forced to fly his country, and repair to one of the cities of refuge, ch. 35: For the same reason David was not allowed to build the temple of God, because he had been a man of war, and had shed much blood, 1 Chronicles 28:3. See Philo, de vita Mosis; and Grotius de Jure B. & P. lib. 2: cap. 24.

It is demonstrable, says Mr. Saurin, that if Moses did not here prescribe particular rules to those who returned from any military expedition, he had already given general ones, of which he now makes the application. See Leviticus 21:1.Numbers 19:4.Leviticus 6:28; Leviticus 15:12.