“It was the avenger’s right and duty to slay the murderer (2 Sam. 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 (Ex. 21:13; Num. 35:13; Deut. 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 [avenger] (Deut. 19:11–13).” Christiananswers.net
God is the ultimate Avenger of blood. He will see to it that every transgression against the moral Law will find just retribution. See Eccles. 12:14.
Joshua 20:9 These were the cities appointed for all the children of Israel and for the stranger who dwelt among them, that whoever killed a person accidentally might flee there, and not die by the hand of the avenger of blood until he stood before the congregation.