Cities of refuge were established so that an unintentional murderer might flee to one of these cities in order that he might live.
Someone who committed murder unintentionally had to remain in his city of refuge. He was not allowed to leave, for by doing so, he would expose himself to the vengeance of his victim's relative, who was legally allowed to kill him. The unintentional murderer was not allowed outside his city of refuge even to save someone else's life.
Similarly, the Torah is our "city of refuge." Inside the Torah and the lifestyle that G-d prescribes for us, we are spiritually alive; if we venture outside the confines of the Torah's lifestyle, we expose ourselves to the risk of spiritual death.
This is true even if it seems that we can save someone's life by making some compromise in the Torah's directives. The Torah is synonymous with life, so only through loyalty to its principles can we both maintain our own spiritual vitality and preserve or enhance the spiritual vitality of others.