Adam Clarke on Exodus 21:13

I will appoint thee a place whither he shall flee - From the earliest times the nearest akin had a right to revenge the murder of his relation, and as this right was universally acknowledged, no law was ever made on the subject; but as this might be abused, and a person who had killed another accidentally, having had no previous malice against him, might be put to death by the avenger of blood, as the nearest kinsman was termed, therefore God provided the cities of refuge to which the accidental manslayer might flee till the affair was inquired into, and settled by the civil magistrate.