"which You have redeemed": It is to this end that You redeemed us — that there be no spillers of blood among us. Variantly: It is to this end that You redeemed us — that if we sinned You would make atonement for us. "and the blood will be atoned unto them":
The Holy Spirit says this: "If you do as prescribed herein, the blood will be atoned unto you."
(Ibid. 9) "And you shall remove the innocent blood from your midst.": Remove the doers of evil from Israel.
(Devarim 21:10) "If you go out to war": Scripture speaks of an optional war (as opposed to a war of mitzvah).