The slayer had to flee to the nearest city of refuge, where the elders would meet him, hear his case, and hold a trial. If it was clear that the slaying was accidental, then the man would be allowed to live in the city under their protection, not to be touched by the avenger. However, if the man left the city, he could be slain. When the high priest died, the man would be free to safely return to his own city. The purpose of this law was to keep the land from being polluted by murderers.
Murder defiles the land, and uncondemned murderers lead the land into greater sin.
This law provided for the protection of the innocent and the condemnation of the guilty. It was a just law. It is too bad that our laws today often make it easy for the guilty to go free. These cities of refuge are beautiful types of Christ. Because of sin, death has passed upon all men.