The coals could be a multiple-meaning or punned illustration. Some scholars see this as a gesture of useful friendship (when fire was hard to strike, people would borrow coals from a neighbor and carry them in an insulated bucket, atop their heads). Some see this as the sense of humility the recipient may experience. Some see this as an analogy to the process of extracting metal from ore. All of these reflect in some way. I don’t think it would mean to literally burn somebody’s head off.
What I believe is it shows that if you treat your enemies as your friends, then they won’t have any reason to hate you any more..................