When this law was re-stated in this week’s portion, three additional cities were designated, making a total of nine instead of six (Deut. 19:9).
According to Rashi, this law applies to the time of the Messiah when the boundaries of Israel would be enlarged, making more cities necessary.
It seems to be a rather strange explanation, bearing in mind that the messianic redemption was due to bring peace and serenity, when there would surely be less killings and not more.
Some of the commentators (e.g. the Meshech Chochmah) take Rashi’s comment as applying to the possibility of Mashi’ach coming early – not such a strange possibility when we consider that we have long feared the opposite, a situation in which he would come late (in the words of one writer, that “he would come when he was no longer necessary” – a view with which some might well disagree, though history is full of Jewish complaints that Mashi’ach is taking his time and meanwhile we are suffering).
But if Rashi is right, the rule in the Torah applies to an interim period whilst we are still in process of preparing the earth for the messianic moment – an interim period in which killing has not yet been finally eradicated, and provision has to be made in case unintentional killings still take place.