ἔκδικος

Romans 13:4 ἔκδικος an avenger

For he is God’s servant for your good. But if you do wrong, be afraid, for he does not carry the sword in vain. He is God’s servant, an agent of retribution to the wrongdoer.

1 Thessalonians 4:6 ἔκδικος avenging [is]

and no one should ever violate or exploit his brother in this regard, because the Lord will avenge all such acts, as we have already told you and solemnly warned you.