If your enemy is hungry, give him food to eat;
if he is thirsty, give him water to drink.
22 In doing this, you will heap burning coals on his head,
and the Lord will reward you.
This verse goes very counter to what we would prefer to do. A first consideration is why is this person my enemy. Sometimes we have enemies because they oppose us even though we are in the right and are innocent. Sometimes we make enemies by how we treat people with our words or actions towards them. I think it is fair to ask if my "enemy" is coming against me in my innocence or if my enemy is coming after me for revenge. If I have done wrong towards someone, I should fix that with apology and repentance.
If someone is coming after us and we are innocent, the desire would be to treat them poorly and stoop to their level. If we see them hungry, we may say "serves them right". If they are thirsty, we may say "good, you get what you deserve.". God tells us to do something radically different. He calls us to compassion and showing kindness to our enemies. He doesn't want us to be just like them. Rather, God wants us to rise above the conflict and do something better.
To heap "hot coals on their head" is not a phrase we use all that often. But when someone is being mean to us and we in turn show kindness, it changes the dynamics in the relationship. Perhaps, it will make them feel a sense of guilt for their own actions as they see your kindness in action. Maybe it is our way of saying I am putting you into God's hands to deal with- I won't seek my own revenge. It certainly opens eyes when instead of treating someone poorly as they may deserve you return their rottenness with kindness. It is a way of honoring God by choosing to do the right thing when being unkind back feels like the natural choice. In a sense we are reminded we deserve nothing good from God, yet while we were still sinners Christ died for us. In light of what Jesus did for his enemies, a little bread and water doesn't seem like much of a sacrifice at all