John Gills Exposition of the Bible Commentary - Deuteronomy 21:9
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 sight of the Lord ; as it was to observe this law concerning the beheading of the heifer, with all the rites and ceremonies... read more
Matthew Henry's Complete Commentary - Deuteronomy 21:1-9
Care had been taken by some preceding laws for the vigorous and effectual persecution of a wilful murderer (Deut. 19:11-13), 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 pollution because it was not through any neglect of theirs that the murderer was unpunished; no, a great solemnity is here provided for the putting... read more