Thou shalt heap coals of fire upon his head - Not to consume, but to melt him into kindness; a metaphor taken from smelting metallic ores: -
So artists melt the sullen ore of lead,
By heaping coals of fire upon its head:
In the kind warmth the metal learns to glow,
And pure from dross the silver runs below.
S. Wesley.

For thou shalt heap (p) coals of fire upon his head, and the LORD shall reward thee.
(p) You will, as if by force, overcome him, in so much that his own conscience will move him to acknowledge the benefits, and his heart will be inflamed.

For thou shall heap coals of fire upon his head,.... Not to increase his punishment and damnation, the more aggravated by kindness shown him; but to bring him by such means to a sense of former injuries, and to shame for them, repentance of them, and love of the person injured, and carefulness for the future of doing him any further wrong;
and the Lord shall reward thee: with good things, for all the good done to thine enemy, whether it has the desired effect on him or not; or whether he rewards thee or not; see Romans 12:20.

For - In so doing, which words are expressed Romans 12:20, where this text is quoted. Thou shalt melt him into repentance, and love.