Albert Barnes Commentary Proverbs 25:21-22

"If thine enemy be hungry, give him bread to eat; And if he be thirsty, give him water to drink: For thou wilt heap coals of fire upon his head, And Jehovah will reward thee." — Proverbs 25:21-22 (ASV)

A precept reproduced by Paul (Romans 12:20) has a second clause that, at first sight, seems to suggest a motive incompatible with true charity. Leviticus 16:12 suggests an explanation. The high priest on the Day of Atonement was to take his censer, fill it with coals of fire, and then put the incense on it for a sweet-smelling savor.

So it is here. The first emotion in another, caused by the good done to him, may be one of burning shame. However, this shame will do its work: the heart also will burn, and prayer, confession, and thanksgiving will rise as incense to the throne of God. Thus, we shall overcome evil with good.