In Romans 12:20, St. Paul quotes Proverbs 25:21-22:
“If your enemy is hungry, feed him; if he is thirsty, give him something to drink. In doing this, you will heap burning coals on his head.” The bit in Proverbs, goes on "and the Lord will reward you".
What are the burning coals all about? One interpretation - it's about hell, and vengeance. If you treat your enemy well, God will ensure he gets his come-uppance. Not at all sure how that fits with mercy and forgiveness. Another thought - the enemy is shamed by the good treatment he receives (and is more likely to reform?). Neither very satisfactory. Any ideas anyone?