The avenger of blood is related to the kinsman-redeemer. Indeed, the same Hebrew word translated as avenger
is translated elsewhere as redeemer.
This person had obligations to fulfill in relation to his family with regards to marriage (Ruth 4:3–6), purchasing property, and helping those in need (see Leviticus 25:25–55).1
This person (the avenger of blood) was a male relative of the deceased (see Leviticus 25:48–49) and he had the responsibility of killing the person who killed his family member if that person did not go to a city of refuge for asylum (Numbers 35:26–28) or if he was found guilty of murder despite having gone to a city of asylum (Numbers 35:16–21). These laws were given at a time when murder was not regulated by political courts but local families.2 Though it might seem as if this was a private vengeance, in reality this was the means by which God’s justice was practically realized. Blood pollutes the land, and no atonement can be made for the land for the blood that is shed in it, except by the blood of the one who shed it
(Numbers 35:33; see also Genesis 9:6). The avenger/redeemer of blood thus had an important legal responsibility of carrying out God’s judgment upon those who have taken the life of another.3
3 that the manslayer who strikes any person without intent or unknowingly may flee there. They shall be for you a refuge from the avenger of blood.