Rashi on Numbers 35:25/רש"י על במדבר ל״ה:כ״ה

1עד מות הכהן הגדול [HE SHALL ABIDE IN IT] UNTIL THE DEATH OF THE HIGH PRIEST — because he serves to cause the Shechinah to dwell in Israel and thereby prolong their days, whilst the murderer serves to make the Shechinah depart from Israel and thereby shortens the days of the living. He is therefore not worthy that he should stand before a High Priest (that he should be anywhere near a High Priest) (Sifrei Bamidbar 160:9). Another explanation why he had to remain there until the High Priest’s death: Because the High Priest should have prayed that this misfortune might never happen in Israel in his days (cf. Makkot 11a).

2אשר משח אתו בשמן הקדש WHO WAS ANOINTED WITH THE HOLY OIL — According to its plain sense, this is one of the elliptical sentences — for it does not expressly mention who anointed him, but it is the same as אשר משחו המושח בשמן המשחה “… the High Priest whom he who carried out the anointing had anointed with the holy oil”. — Our Rabbis, however, expounded it in Treatise Makkot 11b as a proof for a law: that it intends to teach that if, before the sentence on him has been passed that he should go into a city of refuge, the High Priest died and they appointed another in his stead, and afterwards his case was finished, he returns home through the death of the second, because it is said, אשר משח אתו. But did he anoint the priest or did the priest anoint him? for the words may literally mean either: “until the death of the High Priest whom he (the murderer) had anointed” or “until the death of the High Priest who has anointed him (the murderer)” — but the words are used to include the case of the High Priest who was anointed in his days (and whom, as it were he had anointed), viz., that he (such a High Priest) causes him to return home through his death.