Jamieson, Fausset and Brown on Numbers 35:22-28

22-28. But if he thrust him suddenly without enmity, or have cast upon him any thing without laying of wait, &c.--Under the excitement of a sudden provocation, or violent passion, an injury might be inflicted issuing in death; and for a person who had thus undesignedly committed slaughter, the Levitical cities offered the benefit of full protection. Once having reached the nearest, for one or other of them was within a day's journey of all parts of the land, he was secure. But he had to "abide in it." His confinement within its walls was a wise and salutary rule, designed to show the sanctity of human blood in God's sight, as well as to protect the manslayer himself, whose presence and intercourse in society might have provoked the passions of the deceased's relatives. But the period of his release from this confinement was not until the death of the high priest. That was a season of public affliction, when private sorrows were sunk or overlooked under a sense of the national calamity, and when the death of so eminent a servant of God naturally led all to serious consideration about their own mortality. The moment, however, that the refugee broke through the restraints of his confinement and ventured beyond the precincts of the asylum, he forfeited the privilege, and, if he was discovered by his pursuer, he might be slain with impunity.