This isn't a verse saying your enemies will be punished and condemned when you are nice to them. These aren't the fiery coals of Hell. You aren't secretly damning your enemies in some sort of sinister plot. Rather a few commentaries on Proverbs 25:21-22 point out that burning coals is a smelting metaphor, a process of obtaining the valuable metals out of ore. These burning coals have a refining effect of removing the impurities from the metal and leaving only what is valuable. This is a beneficial effect. Our enemies will be ashamed for they have nothing bad to say about us (Titus 2:6-8) and may turn into friends.
But if while we were enemies with God, while we were still sinners Christ died for us so that we may be reconciled to him in death, how much more will we be saved through his life! (Rom 5:1-11) God himself has continued to provide for us though we despised and hated him. How foolish have we become to bite the hand that feeds! But now that God's love has been made plain to us through the death and resurrection of Christ, we now seek to imitate his love. We who have experienced God's love and provision are being refined in the fire, being purified unto peace with a Holy God.