Free translation from a talk of the Rebbe, 18th Elul, 5710 (1950), (excerpt)

Let us first understand the function of the Cities of Refuge, לנוס שמה כל מכה נפש בשגגה - “to which one who unintentionally strikes [i.e., kills] a soul can flee.”

By way of introduction, it will be noted that among the nations of the world there is punishment by imprisonment. In contrast, this is not to be found in the Torah, of which it is written, “For this is your wisdom and your understanding in the eyes of the nations.” (In certain situations a person is locked in a cell, but this is not punishment by imprisonment: it is a preparatory stage towards a different penalty [viz., capital punishment.)

The nations of the world know of (and have) one world only - this world. Here a man lives seventy or eighty years, as in the verse, “The years of our life number seventy, or, if in great vigor - eighty”; at most, “[Man’s] days shall be a hundred and twenty years.” Accordingly, when such a man sins and breaks the law, causing harm to the world (the only world that exists), he is arrested and imprisoned in isolation so that he will not cause the world any further harm.

In this way he is deprived of the freedom that he would need to continue with his tasks. As a result he is automatically deprived of any further connection with the entire pattern of reward and punishment, and with the ultimate purpose of his creation, for the rest of his life. He loses his identity as a human being. Nevertheless, this is regarded as unavoidable in order to protect the public against harm, and the individual, weighed against the public interest, is jettisoned.

In contrast, according to the Torah - which is “a Torah of life” and “a Torah of lovingkindness,” and which is “good for heaven and for humanity” - it is out of the question to sacrifice one cause for another, even to sacrifice an individual and to harm him for the sake of protecting the public against harm.

(To express this principle in the language of the nigleh of the Torah: כל קבוע כמחצה על מחצה ואינו בטל ברוב.)

The Torah does not cause harm, nor are the punishments of the Torah intended merely to punish, but to rectify the sin involved. Punishment by imprisonment is therefore out of the question, because the offender is deprived of the ability to fulfill his role in life.

It is written, ימים יוצרו ולו אחד בהם : “The days [of each man] are fashioned” in a number which the Heavenly Court determines is sufficient to enable him to fulfill his role and his mission, and every single moment of his life should be utilized for this purpose. This is what makes punishment by imprisonment unthinkable.

In every possible situation, even if a man has sinned and transgressed, he remains obliged to continue his mission in life. [If this appears to be problematic, note the words of the Gemara:] בהדי כבשי דרחמנא למה לך - “What concern of yours are the mysteries of G‑d?” In this spirit the Rebbe Rashab writes that today we cannot plumb the depths of the mystic principle of the beirurim. Hence, regardless of one’s current spiritual situation, one should observe every mitzvah that presents itself, without calculating (for example) whether it is proper that he should observe a law stated in Choshen Mishpat when the [earlier, daily] laws in Orach Chayim are still imperfectly observed.

(Capital punishment differs from other forms of punishment: (a) It is not that the existence of one man is being sacrificed for the public good, but that this man, as an individual, is himself deserving of capital punishment, for his own rectification (tikkun); (b) this punishment does not deprive him of an opportunity to pursue a mission in life, because once the Torah (and therefore the Heavenly Court) determines that he should no longer live, he no longer has a role or mission.)

From the above, it is clear that when “one who unintentionally strikes a soul” has to live in one of the Cities of Refuge, this is not punishment by imprisonment. (For even when he is in a state in which the blood avenger can kill him he is still under obligation to continue with his life-work, and this in itself rules out imprisonment.) Rather, living in a City of Refuge is punishment by exile. In the words of Sefer HaChinuch, “so that he will suffer the pangs of exile… [that are felt] when a man is separated from those who love him and from his native land….” At the same time, however, he must be provided with everything that he needs to carry out his mission like a free man. In fact, “if a scholar is exiled, his mentor is to be exiled with him”: even if he is an unworthy scholar, his mentor is nevertheless exiled with him so that he will be able to study Torah from him as in the past.