Adam Clarke on Numbers 35:31

Ye shall take no satisfaction for the life of a murderer - No atonement could be made for him, nor any commutation, so as to save him from death. All the laws of the civilized world have either adjudged the murderer to death, or to a punishment equivalent to it; such as perpetual imprisonment, in a dungeon, under ground, on a stone floor, without light, and to be fed on a small portion of bread and water. In such circumstances a man could live but a short time; and though it is not called the punishment of death, yet, from its inevitable consequences, it only differed from it by being a little longer respite than was usual where the punishment of death was awarded. See the note on Gen 9:6.