But if the manslayer at any time goes beyond the border of his city of refuge to which he may flee, and the blood avenger finds him outside the border of his city of refuge, and the blood avenger kills the manslayer, he will not be guilty of blood because he should have remained in his city of refuge until the death of the high priest. But after the death of the high priest the manslayer shall return to the land of his possession (Numbers 35:26-28).
The Scriptures do not give us a direct answer to this question. However, I have discovered something interesting from ancient Jewish literature and commentaries that discuss this issue, particularly from the Halakhah1 Makkoth 11B. There we find this interesting commentary:
Do you think it is banishment that procures atonement [remission of exile]? It is the
death of the [high] priest that procures the atonement.2
This ancient Jewish commentary suggests that the reason the man slayer had to wait for the death of the high priest was that this death affected atonement for the manslayer. Now, "atonement" is defined in the editorial brackets as "remission of exile," rather than how we traditionally understand it as "remission of sin."
Now, the commentary goes on to find fault with the high priest by suggesting that the high priest failed to plead to God for mercy on behalf of the accidental manslayer:
What should the high — priest [the latter] have done [to avert the unhappy event]? He
should have implored Divine mercy for the slayer's acquittal, which he [seemingly] failed to do.2
I find all this interesting, because we are told in the New Testament, particularly in the Book of Hebrews, that Jesus is our High Priest, whose death procured our atonement, that is, the forgiveness from our sins:
But when Christ came as high priest of the good things that are now already here... He did not enter by means of the blood of goats and calves; but he entered the Most Holy Place once for all by his own blood, thus obtaining eternal redemption. The blood of goats and bulls and the ashes of a heifer sprinkled on those who are ceremonially unclean sanctify them so that they are outwardly clean. How much more, then, will the blood of Christ, who through the eternal Spirit offered himself unblemished to God, cleanse our consciences from acts that lead to death, so that we may serve the living God! (Hebrews 9:11-15).
As the death of the high priest in the Old Testament removed blood guilt, so the death of Jesus, our High Priest, removes all guilt. But as others have noted on this site, the atonement that we receive from Christ is greater because His death covers even our willful sins. What is more, there is no waiting for the the death of the high priest, for He has already died and rose again, and His death was once for all. As we read in the Scriptures,
For by one sacrifice he has made perfect forever those who are being made holy... And where these have been forgiven, sacrifice for sin is no longer necessary (Hebrews 10:14-18).
Therefore, while it is only conjecture based on ancient rabbinical commentary, perhaps the reason why the man slayer had to wait until the high priest died before obtaining full release from guilt, enabling him to return to his home and life, was because the death of the high priest affected a kind of atonement, even as the animal sacrifices affected a kind of atonement. And both of these things foreshadowed the true atonement and enduring forgiveness of sin, which could only come through our eternal High Priest, Jesus Christ.
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1 What was the Halakhah?
Halakhah constituted the practical application of the 613 mitzvot ("commandments", singular: mitzvah) in the Torah, (the five books of Moses, the "Written Law") as developed through discussion and debate in the classical rabbinic literature, especially the Mishnah and the Talmud (the "Oral law"), and as codified in the Mishneh Torah or Shulchan Aruch(the Jewish "Code of Law".) (From Wikipedia halakha).
2 Read the entire section of the Halakhah Makkoth (2a-24b). The sections cited are from Makkoth 11b.