If you think about it, there are positives and negatives to being a Kohain, Levi, or Yisroel:
- A Kohain is closest of the three groups to the shechina, but must consequently follow many limitations in his life outside the Beit HaMigdash (avoid exposure to corpses, marry only a virgin or widow, protect his wife at all costs, etc.).
- A Levi has no actual claim to the land per se, but rather lives in one of the designated Levite cities. Moreover, the Levi performs the "grunt" work in the Beit HaMigdash, so while closer to the shechina than a Yisroel, the Levi is carries a rather "in limbo" status.
- A Yisroel is farthest in his daily life from the shechina, but is consequently freer than the other groups to live his life, albeit al pi halacha.
Because at the end of the day, we are all responsible for one another. The two "holiest" groups must make restitution, so to speak, for the fact that their spiritual negligence brought about a murder. If the Kohain Gadol had been perfect in his duties, then murder would be eradicated from the land, which is why the murderers must remain in the ariyeh miklot until his death. Similarly the Leviim act as a "gateway" to the Kohainim; they are the "filling" in the status sandwich of Bnai Yisroel. Therefore, as the more accessible branch of shevet Levi, they are responsible for rectifying the spiritual impurity that is upon the land due to their tribemates shortcomings.
The common denominator between ariyeh miklot and the unsolved murder is the recognition that we are all interconnected, that one person's wrongdoing affects the entire klal. The elders of the city must measure from the boundary of their city to the corpse to establish that their city was further from the corpse, thereby absolving them of guilt. But what is the guilt being referenced here? That they were insufficient hosts to the deceased, which resulted in lack of protection, and in turn his death.
To put it differently, both mitzvahs bring home the fact that each one of us must always remember that we are all responsible for the welfare of our brethren, and that when our brother fails, his shortcoming impacts us all in some way: spiritually, economically, emotionally, physically. It is of the utmost importance then, that we try to help each other stay on the correct path, to make the right choices- to live as proper yiddin. Since each group has their specified purpose, we must ensure that each group acts in accordance with the mitzvahs associated with that purpose if we as a people are to enjoy continued spiritual success.