avenger of blood
It was the avenger’s right and duty to slay the murderer (2 Samuel 14:7, 11) if he found him outside of a city of refuge. In order that this law might be guarded against abuse, Moses appointed six cities of refuge (Exodus 21:13; Numbers 35:13; Deuteronomy 19:1, 9). These were in different parts of the country, and every facility was afforded the manslayer that he might flee to the city that lay nearest him for safety.
The avenger was forbidden to follow him into the city of refuge. This arrangement applied only to cases where the death was not premeditated. The case had to be investigated by the authorities of the city, and the willful murderer was on no account to be spared. He was regarded as an impure and polluted person, and was delivered up to the goel (Deuteronomy 19:11-13).