Verse Nu35-25a states And the congregation shall deliver the negligent murderer from the hand of the avenger of blood, and the congregation shall restore him to his city of refuge, where he had fled; and he shall live there until the death of the high priest, who was anointed with the holy oil.

Rashi comments on the connection between the underlined phrase connecting the death of the high priest to the release of the negligent murderer from the prison city of refuge:A primary task of the Priest is to atone for inadvertency. For example a person who negligently descecrates the Sabbath receives atonement by bringing a sin offering and attending to the procedures performed by the priest. If the priests had done their job perfectly there would be no negligent murders. Consequently when the High Priest dies ( as a punishment for lack of prevention of negligence) the negligent murderer goes free.

We think the above Rashi perfectly clear. We further add that the general rules governing sybolism may be found in my article Genesis 1 speaks about the creation of Prophecy not the Creation of the world which can be found on the world wide web at http://www.RashiYomi.com/gen-1.htm .
• In that article we have explained that there is an obligation to interpret a commandment as symbolic when the Bible relates two items that are superficially unrelated. A typical example occurs in our salute to a flag. A flag by itself is an item totally unrelated to the country we live in. Therefore our salute to the flag is seen as a symbolic gesture affirming our affiliation to our country.
• We have also explained in the above article that when we are obligated to interpret a commandment symbolically we do so by using the function, form, linguistic expressions of the item. Hence in the example today we interpret the Priest in terms of his function -- to prevent negligence.

We should explain why the death of the priest releases the prisoner. Prior to the death of the priest the blood-avenger blaimed the murder on the murderer's negligence. The blood-avenger may wish to avenge the murder by killing the murderer. However when the High Priest dies a message is sent to the blood avenger: Perhaps the murder is not the murderer's fault. Perhaps it is the priest's fault. If the priests had been more diligent in their prevention of negligence then the murder would not have happened.Since you are not certain whether the negligent murder was the fault of the priest or the murderer you shouldn't want to kill the murderer.