When someone has wronged you, you are often told to turn the other cheek or to be nice to your persecutor so that “burning coals may be heaped upon their head.” In other words, you are to be nice to them and smile at them so that God will eventually punish them and then give you a pat on the head for being such a good little Christian for taking the high road.
I think that sometimes this verse (Proverbs 25:22, Romans 12:20) is viewed, whether admittedly or not, as an apology for passive-aggressive behavior when dealing with those we disagree with or don’t like. We can passively smile, nod, and do good to our enemies while hating them in our heart and waiting for divine judgment that will declare them wrong and us right. So heap those burning coals and may they burn like hell-fire upon your enemies head.
Look, these verses are not referring to a sanctified view of karma; of what goes around comes around. Give heed to Martin Luther from his Romans Commentary on verse 12:20:
“Blessed Augustine says: “We must understand this expression in the sense that we induce him who has injured us to repent of his action, and thus we benefit him.” For these “coals” (that is, benefits) have the power to burn, that is, to distress, his spirit. The psalmist speaks of this in Ps. 120:4: “The sharp arrows of the mighty, with coals that lay waste.” Thus God also converts those whom He converts with a view of His goodness. And this is the only way to achieve a true conversion, namely, through love and kindness. For he who is converted through threats and terror is never truly converted as long as he retains that form of conversion. For fear makes him hate his conversion. But he who is converted by love is completely burned up against himself and is far more angry with himself than anyone else can be with him, and he is totally displeased with himself. For such a person there is no need for prohibition, for being under surveillance, and for making satisfaction. For love teaches him all things; and when he has been touched by love, he will exhaust himself in seeking out the person whom he has offended.
And thus the good deeds shown toward our enemies are the “burning coals,” not those shown to our friends. For a friend does not feel about a good deed the way an enemy does, for he assumes that he has the right to expect good deeds and never receives enough, nor is he surprised at the kindness of his benefactor. But an enemy, because he realizes that he does not deserve an act of kindness, is completely captivated by his benefactor. In the same way God gave His only-begotten Son for His enemies, so that He might make us burn with the warmest love toward Him and that He might bring about in us the greatest possible hatred for ourselves. Christ is the furnace filled with fire, as we read in Is. 31:9: “Thus says the Lord, whose fire is in Zion, and whose furnace is in Jerusalem.” We heap “burning coals” on the head of our adversary, but God heaps the Fire itself upon him.”
The purpose of burning coals being heaped upon your enemy’s head (your good deeds toward them) is that they will repent and turn to the Lord, which is for their benefit. It is not so that you can stand in judgment over them and help the Lord punish them. The will of the Lord is that all men come to the knowledge of salvation and receive the forgiveness of sins that Christ won for all mankind. To love your brother and your enemy, repent of your hatred for them and pray that they remain in the Lord’s mercy or turn to it. By repenting of your own sins and forgiving and desiring forgiveness for those who trespass against you, the Lord will give you peace.
 Luther, M. (1999). Luther’s works, vol. 25: Lectures on Romans (J. J. Pelikan, H. C. Oswald & H. T. Lehmann, Ed.) (Ro 16:27). Saint Louis: Concordia Publishing House.