Servant: The New Testament often uses this Greek word (diakonos) to describe Christians serving God in different roles. In non-religious Greek, diakonos can also refer to a city official. A government and its leaders serve God by administering justice, whether they realize it or not.
he does not carry the sword in vain: Here, the sword might be a metaphor for any punishment. Some interpreters believe it means human governments, under God's authority, have the right to use capital punishment. (Capital punishment is the practice of killing someone as a legal penalty for certain crimes.)