Romans 12:20, a quotation of Proverbs 25:21,22, has occasioned much debate and given rise to multiple interpretations. The problem is that the most natural explanation seems to be that you should do good to your enemies so that they may suffer an increase of punishment from God. But this interpretation contradicts itself. If we perform acts of kindness to our enemies with the hope of increasing their punishment, then we are, at least in our hearts, taking the vengeance that the Lord forbids in verse 19.
Besides this interpretation, Charles Hodge mentions two others in his commentary on the passage. The first is that in doing good you cause your enemy to suffer the pain of shame and remorse. The second is that kindness will subdue your enemy, as fire subdues metal to profitable shaping. You will overcome evil with good, as verse 21 says. The problem with both is that the Scriptures usually use coals as figures for judgment (cf. Psalm 18:12, 120:4, 140:10, Ezekiel 10:2).
I would like to suggest another interpretation that relates the verse to the words of verse 19: “give place to wrath.” Though there is disagreement also about the meaning of that command, the idea is probably that you must give place to the wrath of God. You must allow that wrath to accomplish vengeance on your behalf.
When we take vengeance ourselves, we get in the way of God’s vengeance. Our imperfect, often hateful, proud and cruel vengeance prevents the working out of God’s righteous vengeance. Because we insist on taking vengeance ourselves, God restrains his own vengeance. The thought is the reverse of Paul’s instruction in 1 Corinthians 11:32: “For if we would judge ourselves, we would not be judged.” There Paul encourages us to take action against ourselves, so that we do not come under the judgment of God. Here he says, “Do not take action yourselves. Make room for God to act.” Thus, too, you will overcome evil with good. God’s vengeance may or may not convert your enemy, but, whatever it does, it will effectively and permanently overcome his evil.
The difficulty with this interpretation is that the apostle says, “You will heap coals of fire on his head. The reason for putting it that way may be that your kindness to your enemy is another reason for God to avenge you speedily. In fact, your kindness and love will become an instrument of vengeance for punishment or correction in God’s hands, as God uses them to set him in a slippery place (cf. Psalm 73:18). You do good to him, so that God may fully work his purpose regarding him.
This is another powerful argument against personal vengeance. When we take our own vengeance, we get in the way of God’s vengeance and, at least temporarily, interfere with the working of his justice. The message is, “Stand aside, and thus make room for God to heap on the head of your enemy whatever coals of fire he considers necessary. He knows best how to avenge you.”