Makkos, Chapter 2, Mishnah 7. This mishnah addresses the status of the accidental killer when there is no Kohen Gadol, and how the killer leaves the city of refuge.
One who never leaves the city of refuge:
The mishnah lists three cases in which the accidental killer never leaves the city of refuge:
His verdict was concluded at a time when no Kohen Gadol was appointed.
"HaHoreg Kohen Gadol" - one who killed a Kohen Gadol.
"Kohen Gadol shehareg" - a Kohen Gadol who himself killed accidentally.
Regarding the latter two cases the Rishonim disagree: the Rambam explains that the mishnah is speaking of a case in which no other Kohen Gadol was appointed at the time the verdict was concluded. The Ritva, however, holds with regard to one who killed a Kohen Gadol that even if another Kohen Gadol was appointed, since he killed a Kohen Gadol he does not leave.
He does not leave for any need whatsoever:
The mishnah continues and establishes that an accidental killer residing in a city of refuge does not leave it for any need at all:
"Lo le'eidus mitzvah" - not to give testimony involving a mitzvah, such as when he saw the new moon.
"Velo le'eidus mamon" - not to testify in monetary cases.
"Velo le'eidus nefashos" - not to testify in capital cases.
"Va'afilu Yisrael tzerichim lo" - even if all of Israel needs him.
"Va'afilu sar tzeva Yisrael keYoav ben Tzeruyah" - even if he is a general whose services are needed.
Nevertheless, "eino yotzei misham le'olam" - he never leaves there, as it says: "asher nas shamah" - the place to which he fled, that is the city of refuge:
"Sham tehei diraso" - there he must live.
"Sham tehei misaso" - there he must die.
"Sham tehei kevuraso" - there he must also be buried.
Moreover, even one who died after his verdict was concluded, before he managed to go into exile, is taken and buried in the city of refuge.
The city's boundary:
"Ucheshem she'ha'ir koletet, kach techumah kolet" - not only does the city itself protect the murderer, but so does its surrounding area, within the two thousand cubits around it.
"Rotzeach sheyatza chutz latechum u'metza'o goel hadam" - a murderer who left the boundary and was found there by the blood avenger. The Tannaim disagree about his status:
Rabbi Yosi HaGelili: "Mitzvah beyad goel hadam" - it is a mitzvah for the blood avenger to kill him; "reshut beyad kol adam" - and any other person is permitted to kill him, though he is not obligated to do so.
Rabbi Akiva: "Reshut beyad goel hadam" - the blood avenger is permitted to kill him but is not commanded to do so; "vechol adam eino chayav alav" - another person who killed him is not liable to death for killing him, but he has no permission to kill him.
It is worth noting that there is another version of the Mishnah: "vechol adam chayav alav" - meaning, on the contrary, that any other person who killed him is liable for killing him.
The Gemara points out that the ruling of the Mishnah, that the murderer is killed for leaving the boundary of the city of refuge, applies specifically to one who left intentionally. But one who left unintentionally is not killed, since it cannot be that his situation would be more severe than the law was to begin with: one who kills unintentionally is exiled and is not killed, and all the more so he should not be killed for leaving unintentionally.
A tree standing on the edge of the boundary:
The Mishnah continues and discusses what is included within the boundary: a tree standing inside the boundary whose branches spread out beyond the boundary, or a tree whose trunk stands outside the boundary and whose branches spread into the boundary - "hakol holech achar mekom hanof" - everything follows the location of the foliage.
The Gemara explains that the intent is to include both the trunk and the foliage, and in both directions: if the branches are inside the boundary and the trunk is outside the boundary, one standing beside the trunk is saved on account of the foliage; and if he is standing beside the foliage that is outside the boundary, he is saved on account of the trunk that is inside the boundary.
One who kills within the city of refuge itself:
If he killed while he was in that very city of refuge - he is exiled from one neighborhood in the city to another neighborhood. And likewise a Levite who killed, who lives in a city that is itself a city of refuge - he is exiled from one neighborhood to another.
In summary: in this Mishnah we learned the three cases in which the murderer never leaves the city of refuge - when his case was concluded at a time when there was no Kohen Gadol, one who killed a Kohen Gadol, and a Kohen Gadol who killed; the rule that he does not go out for any testimony or for any public need, and that even his death and burial are there; the law that the boundary provides refuge just like the city, the dispute between Rabbi Yosi HaGelili and Rabbi Akiva regarding one who leaves the boundary, and the distinction between leaving intentionally and leaving unintentionally; the law of a tree standing on the edge of the boundary, and the exile of one who kills within the city of refuge itself from one neighborhood to another.