A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
num 35:2EIGHT AND FORTY CITIES GIVEN TO THE LEVITES. (Num 35:1-5)
give unto the Levites of the inheritance of their possession cities to dwell in--As the Levites were to have no territorial domain allocated to them like the other tribes on the conquest of Canaan, they were to be distributed throughout the land in certain cities appropriated to their use; and these cities were to be surrounded by extensive suburbs. There is an apparent discrepancy between Num 35:4 and Num 35:5, with regard to the extent of these suburbs; but the statements in the two verses refer to totally different things--the one to the extent of the suburbs from the walls of the city, the other to the space of two thousand cubits from their extremity. In point of fact, there was an extent of ground, amounting to three thousand cubits, measured from the wall of the city. One thousand were most probably occupied with outhouses for the accommodation of shepherds and other servants, with gardens, vineyards, or oliveyards. And these which were portioned out to different families (Ch1 6:60) might be sold by one Levite to another, but not to any individual of another tribe (Jer 32:7). The other two thousand cubits remained a common for the pasturing of cattle (Lev 25:34) and, considering their number, that space would be fully required.
num 35:6CITIES OF REFUGE. (Num 35:6-8)
there shall be six cities for refuge, which ye shall appoint for the manslayer--The establishment of those privileged sanctuaries among the cities of the Levites is probably traceable to the idea, that they would be the most suitable and impartial judges--that their presence and counsels might calm or restrain the stormy passions of the blood avenger--and that, from their being invested with the sacred character, they might be types of Christ, in whom sinners find a refuge from the destroyer (see Deu 4:43; Jos 20:8).
num 35:8the cities which ye shall give shall be of the possession of the children of Israel--The burden of furnishing those places for the residence and support of the Levitical order was to fall in equitable proportions upon the different tribes (see Num 33:54; Jos 20:7).
num 35:11THE BLOOD AVENGER. (Num. 35:9-34)
that the slayer may flee thither, which killeth any person at unawares--The practice of Goelism, that is, of the nearest relation of an individual who was killed being bound to demand satisfaction from the author of his death, existed from a very remote antiquity (Gen 4:14; Gen 27:45). It seems to have been an established usage in the age of Moses; and although in a rude and imperfect state of society, it is a natural and intelligible principle of criminal jurisprudence, it is liable to many great abuses; the chief of the evils inseparable from it is that the kinsman, who is bound in duty and honor to execute justice, will often be precipitate--little disposed, in the heat of passion or under the impulse of revenge, to examine into the circumstances of the case, to discriminate between the premeditated purpose of the assassin and the misfortune of the unintentional homicide. Moreover, it had a tendency, not only to foster a vindictive spirit, but in case of the Goel being unsuccessful in finding his victim, to transmit animosities and feuds against his descendants from one generation to another. This is exemplified among the Arabs in the present day. Should an Arab of one tribe happen to kill one of another tribe, there is "blood" between the tribes, and the stain can only be wiped out by the death of some individual of the tribe with which the offense originated. Sometimes the penalty is commuted by the payment of a stipulated number of sheep or camels. But such an equivalent, though offered, is as often refused, and blood has to be repaid only by blood. This practice of Goelism obtained among the Hebrews to such an extent that it was not perhaps expedient to abolish it; and Moses, while sanctioning its continuance, was directed, by divine authority, to make some special regulations, which tended both to prevent the unhappy consequences of sudden and personal vengeance, and, at the same time, to afford an accused person time and means of proving his innocence. This was the humane and equitable end contemplated in the institution of cities of refuge. There were to be six of these legalized asyla, three on the east of Jordan, both because the territory there was equal in length, though not in breadth, to Canaan, and because it might be more convenient for some to take refuge across the border. They were appointed for the benefit, not of the native Israelites only, but of all resident strangers.
num 35:16If he smite him with an instrument of iron, so that he die, &c.--Various cases are here enumerated in which the Goel or avenger was at liberty to take the life of the murderer; and every one of them proves a premeditated purpose.
num 35:22But 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.
num 35:29these things shall be for a statute of judgment unto you throughout your generations--The law of the blood-avenger, as thus established by divine authority, was a vast improvement on the ancient practice of Goelism. By the appointment of cities of refuge, the manslayer was saved, in the meantime, from the blind and impetuous fury of vindictive relatives; but he might be tried by the local court, and, if proved guilty on sufficient evidence, condemned and punished as a murderer, without the possibility of deliverance by any pecuniary satisfaction. The enactment of Moses, which was an adaptation to the character and usages of the Hebrew people, secured the double advantage of promoting the ends both of humanity and of justice.