Cities of refuge

God commanded the Israelites to set aside six cities devoted to sheltering people who have killed by accident. (check out Deutoronomy 19) I guess it must have been a relatively common problem, if the six cities were populated.....

How do we translate this into modern day terms? In a sense, we provide shelter for all people who kill, either intentionally or otherwise, in the form of our gaols. The motives seem to differ however. Biblically, it was to prevent him being killed in revenge, since he was innocent. Modern day rationale says rather that it is for the protection of society in general, in case it happens again, and in punishment.