For thou shalt heap coals of fire upon his head, and the LORD shall reward thee.
For thou shalt heap coals of fire upon his head - melting him into sorrow, pain, and shame, at having been thine enemy. Either be shall be like wax melted by fire, or like clay hardened by it. In either case, "the Lord shall reward thee."
The Lord shall reward thee. Even if thy love fail to melt him, thy labour will not be lost; it will redound to thy good (cf. Psalms 35:13; Luke 10:5).