So you shall purge from among you the guilt of shedding innocent blood, since you have done what is right in the eyes of the LORD.
Care had been taken by some preceding laws for the vigorous and effectual persecution of a wilful murderer (Deu 19:11 etc.), the putting of whom to death was the putting away of the guilt of blood from the land; but if this could not be done, the murderer not being discovered, they must not think that the land was in no danger of contracting any...
Commenting on Deuteronomy 21:1-9
So shalt thou put away the guilt of innocent blood from among you,.... Which otherwise, the person not being found out, and brought to just punishment for it, would devolve upon the whole. Aben Ezra interprets it the punishment of innocent blood, which, by the above method being taken, would not be inflicted on them: when thou shalt do that which is right in the...
Deu 21:9 In this way Israel was to wipe away the innocent blood (the bloodshed) from its midst (cf. Num 35:33). If the murderer were discovered afterwards, of course the punishment of death which had been inflicted vicariously upon the animal, simply because the criminal himself could not be found, would still fall upon him.