Makkot 11 - Unintentional Killer and the High Priest
One whose sentence was finalized when there was no High Priest, one who accidentally kills a High Priest, or a High Priest who killed - these never leave the city of refuge.
One may not leave the city of refuge for testimony regarding a mitzvah, a monetary dispute, or capital punishment. Even if all Israel needs him, and he is like the general Yoav, the son of Zeruyah, he never leaves.
If a killer left the boundary of the city of refuge and the redeemer of the blood found him, Rabbi Yose HaGligli says that it is a mitzvah for the redeemer to kill him. Everybody else is allowed to do so, but Rabbi Akiva says that it is permissible for the redeemer, and everybody else is liable to death for killing him.
Art: The Murder of Rizzio by John Opie