(Bamidbar 35:25) "And the congregation shall return him": From here you learn that those who kill, either unwittingly or wittingly, repair to the cities of refuge, and beth-din send and bring them from there. One who is found liable for the death penalty is killed. One who is not found liable is let go. One who is found liable for exile is returned to his place, as it is written "and the congregation shall return him to his city of refuge." "and he shall dwell there until the death of the high-priest": R. Meir says: A murderer shortens a man's days, and the high-priest lengthens a man's days.
It is not fitting that the "shortener" should stand before the "lengthener." Rebbi says: A murderer defiles the land and removes the Shechinah, and the high-priest causes the Shechinah to repose on the land. It is not fitting that he who defiles the land should stand before him who causes the Shechinah to repose upon the land.