Charles Ellicott Commentary Romans 12:20

"But if thine enemy hunger, feed him; if he thirst, give him to drink: for in so doing thou shalt heap coals of fire upon his head." — Romans 12:20 (ASV)

You shall heap coals of fire.—Compare to Psalms 18:12-14, where the phrase “coals of fire” is used of divine vengeance. So here, but in a strictly metaphorical sense, it means, “You shall take the best and most summary vengeance upon him.” There may be the underlying idea of awakening in the adversary the pangs of shame and remorse.