This is probably one of the more puzzling scriptures: are we to feign kindness in order to add to the ultimate suffering of the unbeliever? Surely not!
How about this... First note that it is Paul who is quoting this scripture. If we consider the meaning of "head" in Paul's writings we see that he more often uses it figuratively than literally: figuratively someone's "head" is the person whose authority they are under. This is most obvious in the idea of Christ being the "head" of the church.
So who is the "head" of our enemies? The Devil!
Remember the promise God made him in the garden? "I will put enmity between you and the woman, and between your offspring and her offspring; he shall bruise your head, and you shall bruise his heel." (Gen 3:15)
Our good works are redemptive: not for us, but for those to whom we perform them: so by loving our enemy we draw him to the Kingdom by the love of God in action. Romans 12 continues, "Do not be overcome by evil, but overcome evil with good" (Rom 12:21). Our love in action is a means by which evil is overcome, the kingdom of darkness is driven our and the Kingdom of God comes.
As a result of this process the guilt and righteous punishment of the Devil is magnified - burning coals are heaped upon him.
So love your enemies, and bring condemnation on the evil one!