Be merciful. Be propitious, or make expiation or atonement.
innocent blood. Put by Figure of speech Synecdoche (of Species), for the guilt which shed it. See App-6.
shall be. shall assuredly be.
Related Commentaries of Deuteronomy 21:8
Deuteronomy 21:8
8 Be merciful, O LORD, unto thy people Israel, whom thou hast redeemed, and lay not innocent blood unto thy people of Israel's charge. And the blood shall be forgiven them.