Romans 12:21

μὴ νικῶ : the absence of any connecting particle gives the last verse the character of a summary: in a word, be not overcome by evil. ὑπὸ τοῦ κακοῦ = by the evil your enemy inflicts. The Christian would be overcome by evil if it were able to compel him to avenge himself by repaying it in kind. Wrong is not defeated but doubly victorious when it is repelled with its own weapons; we can only overcome it ἐν τῷ ἀγαθῷ through the good we do to our adversary, turning him so from an enemy into a friend. Vincit malos, says Seneca, pertinax bonitas: Wetst. accumulates similar examples from classical writers. The ἐν in ἐν τῷ ἀγαθῷ is probably = בְּ : it might be explained as instrumental, or rendered “at the cost of”.