Deuteronomy 21:9 - Joseph Benson’s Commentary on the Old and New Testaments

So shalt thou put away the guilt of innocent blood Till this was done, the guilt was to be looked upon as national; but upon this being solemnly performed, the government was deemed to have done its duty, and the nation cleared of all guilt in this matter. No doubt the chief end of the appointment of this ceremony was to beget and preserve in the minds of men an abhorrence of murder, and a care to prevent or detect it.