In the Epistle to the Romans, Paul says that we should not repay evil with evil (12:17), and he also speaks of this in other letters (1 Thess. 5:15). It is certainly not a new truth that Paul reveals, for we already find this in the Hebrew Bible (Prov. 20:22). That latter text also explains why we do not need to repay evil with evil. We know a God who will set right all evil, all injustice.
Romans 12
21 Do not be overcome by (lit: under) evil, but overcome evil in good. the good
What Paul means by the good is evident from the preceding passage in Romans 12. Bless those who persecute you, do not bless or curse (:14), look out for the good of all people (:17), live in peace with all people as far as it depends on you (:18), etc. All these things can be traced back to the recognition that God is truly God: from Him, through Him, and to Him are all things (11:36). God is soon going to put an end to this evil aeon (Gal. 1:4), but we may already know Him now and be changed in our thinking, a thinking that is diametrically opposed to the thinking of this aeon (12:2).
injustice
The latter can evoke frustration. After all, we see so much injustice around us. Some believers are thereby provoked to point out and expose evil. But that, too, is a frustrating matter, for in an evil aeon, there is no end to it. There is only one truth, but there are countless lies. After all, Satan is called: the father of lies (John 8:44).
living in the good
Whoever fights against evil is controlled by evil. Just as the one who fights against sin is controlled by sin (Rom. 6:14). Who is Paul talking about in Romans 6? Unbelievers who simply live their lives in this world, without any awareness of God? No, he is talking there about religious people who are under the law and do everything they can to avoid and fight sin. It is precisely they who are controlled by sin.
It is the same with those who think they must fight evil. They are controlled by evil. How liberating, then, are Paul’s words: do not let evil get you down; that is not the atmosphere in which we live. Conquer evil in the good!