It was only a murderer to whom the tabernacle was to be no protection (margin reference). Hence, the reference to the “innocent blood.”
Related Commentaries of 1 Kings 2:31
1 Kings 2:31
31 And the king said unto him, Do as he hath said, and fall upon him, and bury him; that thou mayest take away the innocent blood, which Joab shed, from me, and from the house of my father.