Adam Clarke on Joshua 20:2

Cities of refuge - An institution of this kind was essentially necessary wherever the patriarchal law relative to the right of redemption and the avenging of blood was in force; we have already seen that the nearest of kin to a deceased person had not only the right of redeeming an inheritance that had been forfeited or alienated, but had also authority to slay on the spot the person who had slain his relative. Now, as a man might casually kill another against whom he had no ill-will, and with whom he had no quarrel, and might have his life taken away by him who was called the avenger of blood, though he had not forfeited his life to the law; therefore these privileged cities were appointed, where the person might have protection till the cause had been fully heard by the magistrates, who certainly had authority to deliver him up to the avenger, if they found, on examination, that he was not entitled to this protection. On this subject see the notes on Num 35:11 to the end.