Proverbs 25: 21-22 says “If your enemy is hungry, give him food to eat; if he is thirsty, give him water to drink. In doing this, you will heap burning coals on his head, and the Lord will reward you.”

Doesn’t it suggest that it’s okay for Christians to wish their enemies would suffer from burning coals as long as they are nice to them on the outside?

It seems this verse is rather misunderstood. As Christians, we shouldn’t seek our own revenge against our enemies. We should instead seek to behave in such a way that the justness of their punishment for their own sins is not reduced by our own sinfulness towards them. In other words, when someone is cruel to you but you still respond with kindness it reveals how dishonorable they are and further justifies the punishment they may receive for their actions. You haven’t attacked them, you’ve allowed them to attack themselves, or to heap these coals on their own heads. But when you respond to their wrongdoing with your own, you reduce the righteousness of them receiving their full punishment in addition to what you've already done against them.

But if this is so, then why does the proverb say it is us who heaps coals on others’ heads? Why doesn't it say, "In doing this, you will allow them to heap burning coals on their own heads"? Well, the concept is similar to when someone talks very badly about you, and then you reveal yourself to the people as a good and honorable person. By doing this, you’ve heavily embarrassed the one who tried to damage your name, but at the same time they are responsible for this embarrassment. This is why Paul can rightly say it is us who heaps burning coals on the heads of our persecutors.

It is clear from other verses in the Bible, such as Proverbs 24:17 and Matthew 5:21-22, that we are not to wish bad things onto our enemies and that we should not use kindness as a weapon against them. Therefore, the best way to heap these coals on our enemies is to use kindness as a weapon against sin only and keep our hopes for them pure. This is very difficult—in fact, it is impossible without God. In addition to communication with Him and meditation in His Words, we should seek to catch ourselves when thinking such things against those who hurt us, even in the context of being kind to them externally.