Post: Why should the accidental killer receive a punishment of exile? And why should he go free when the Kohen Gadol dies? It seems a bit strange, and some reason behind these details would be nice. Chazal have a machlokes whether darshinan taamah dikra. Do we try to find the reason behind mitzvos. Even though we decide against, that is only as it impacts halacha, such that we do not say that where the reason fails, the commandment fails. But it still is a mitzvah of Talmud Torah to engage in such inquiry.
Rambam and Ibn Caspi discuss what the motivation behind these details might be, from (what seems to me to be) a rationalist perspective.
Thus, in Moreh Nevuchim, chelek 3, perek 40, Rambam writes:
"That which the accidental killer is exiled, it is to calm the soul of the blood relative, such that he does not see the one via whom the mishap occurred. And his return is predicated upon the death of the Kohen Gadol, for this is a natural matter for humans, that anyone who misfortune falls, when it impacts one like him, or one greater than him, he finds comfort on what happened to him. And there is no greater occurrence of death by us than the death of the Kohen Gadol."
And Ibn Caspi, in his commentary on Masei, writes:
"Until the death of the Kohen: Hashem wants, because of the righteousness of His laws, that his exile should not be forever. And there is no greater boundary than this, combined with this the reason of Rabbenu Moshe za'l. (Moreh Nevuchim, perek 40 from the third chelek)"
These explanations do make sense, and is consistent but one could imagine other explanations making equal sense.
I recall hearing the following Ancient Near Eastern law, though I have never seen it inside to confirm. That was that when someone killed another person (maybe by negligence), his penalty was having to move into the town and taking over for that other person. I am not sure if this included marrying the fellow's wife. The point being that he deprived the town of their citizen, and so he had to make them whole.
This exile could be cast as a spin on that sort of law. He must take his place in a city of exiles. Or a statement against this idea, that he must be exiled for a time.
I can see this as a reenactment of the punishment of Kayin. He deliberately killed his brother, and his punishment was not being able to settle, but being forced to wander over the face of the earth. And he feared that anyone who saw him would kill him. This was subsequently relaxed, to various degrees. He had a special sign to prevent anyone from killing him, and eventually we hear of him building a city.
On one side, we have the goel hadam who would like to take blood-vengeance. And on the other side we have the fact that this was more or less accidental -- perhaps somewhat negligent. As stated, the law is a middle position between the two. We could say that since it was accidental, and Hashem brought this about to his hands, he should get off scot-free. But he did take a human life, as well as cause harm to the living relatives. And so, he reenacts Kayin's penalty, by going into exile, with the goel hadam serving as a sort of enforcer.
Why go free at the Kohen Gadol's passing? Like this, there is an end, to indicate that the sin was not so grievous that there is no end to the penalty. But more than that, the Kohen Gadol has a ritual role. And so, when he dies, they might consider it as atonement effected on their behalf. As the Yerushalmi in the beginning of Yoma reads:
למה הוא מזכיר מיתתם ביום הכיפורים? ללמדך שכשם שיום הכיפורים מכפר על ישראל, כך מיתתם של צדיקים מכפרת על ישראלAnd of course, this is not due to deliberate human action. Rather, it is the natural order, and Hashem's hand, which brings the death of the Kohen Gadol about, which makes a fitting end for the period of atonement for the accidental murderer, where Hashem brought it to the accidental murderer's hand.