Albert Barnes Commentary Exodus 21:13-14

"And if a man lie not in wait, but God deliver [him] into his hand; then I will appoint thee a place whither he shall flee. And if a man come presumptuously upon his neighbor, to slay him with guile; thou shalt take him from mine altar, that he may die." — Exodus 21:13-14 (ASV)

There was no place of safety for the guilty murderer, not even the altar of Yahweh. Thus, all superstitious notions connected with the right of sanctuary were excluded. Adonijah and Joab (1 Kings 1:50; 1 Kings 2:28) appear to have vainly trusted that the common feeling would protect them if they took hold of the horns of the altar on which atonement with blood was made (Leviticus 4:7).

But for one who killed a man “at unawares”—that is, without intending to do it—the law afterward appointed places of refuge (Numbers 35:6–34; Deuteronomy 4:41–43; Deuteronomy 19:2–10; Joshua 20:2–9). It is very probable that there was some provision corresponding to the cities of refuge, perhaps based upon old usage, in the camp in the Wilderness.