Death of High Priest
The Abarbanel gives a beautiful explanation as to why the accidental murderer is allowed to leave the City of Refuge upon the death of the Kohein Hagadol.
The High Priest was the spiritual leader of the Jewish people. He was the central figure each Yom Kippur who brought about atonement for the Jewish people. Those who followed the path of Aharon, were greatly loved and admired.
When the Kohein Hagadol passed away, it was devastating news for the entire Jewish people. It also shook up the ג׳ואל הדם, the redeemer of blood. He was permitted to kill the one being protected by the City of Refuge, if he ever left the City of Refuge. The killer did not kill his brother intentionally, but was careless, nonetheless.
The realization that death was inevitable for all human beings, including the High Priest, created an awakening for all.
This collective trauma, allowed the avenger of blood to calm down. He no longer felt the urgency to avenge his brother’s death. He realized that eventually, everyone would die.
This change in attitude due to the death of the Kohein Hagadol, allowed the accidental murderer to leave the City of Refuge. He was no longer in danger, and he could continue living life as a free man.