In Jewish theology, only the one sinned against can forgive. If I sin against another man, God cannot forgive that sin, only the man who I sinned against can forgive it. If I murder him, there can be no complete forgiveness because (1) he is no longer here to forgive me and (2) God can't forgive it because the sin wasn't against God, or at least, it wasn't JUST against God.
According to Christian theology, the cities of refuge solution was necessary because mankind was not yet endowed with the ability to love as God loves. Before Christ, we had no access to the full grace of God.
Once Christ comes, He provides for the forgiveness of sin in every situation. That's what the sacrament of confession is about: "Whose sins you retain are retained, what sins you forgive are forgiven." Christ not only insists that God can forgive ANY sin regardless of origin or object, He also insists that this divine power to forgive sins devolves upon His apostles. The divine power to forgive can be wielded by men.
As a result, in Christian society, cities of refuge are no longer necessary, or, perhaps, it is better to say that they have been transformed. Whereas before we fled to a city of refuge, now we flee to the confessional of refuge. Those who have sinned can be really and fully forgiven in every circumstance. Those sinned against are required and empowered by God to forgive. Both sides need only ask for the grace, the power, and it will be received.
Thus, from the Christian perspective, the fact that Zimmerman appears today to need a city of refuge merely demonstrates how far America has wandered from Christian ideals and perspectives.