The Return of the King Themes
In The Return of the King, Tolkien examines leadership, particularly that of kings and similar political figures. Denethor, for example, has used his power to foresee the coming of a vast army, and thereby loses hope, for he does not realize that the coming army is an ally and not an enemy. With that hope lost, Denethor consequently abandons his duty to care for his family and the city of Minas Tirith in its time of greatest need by committing suicide and attempting to put his son and heir Faramir to death as well. Just as Denethor is not a "real king," but a steward of Gondor, so his actions do not, to Tolkien's mind, befit a king. The true king, Aragorn, rather than seeking death as an end to his struggle passes through the Paths of the Dead (and thus symbolically through death itself) to take his rightful place on the throne of Gondor. Denethor passes from life into death, but does not have in himself the power to return from the dead, even for the sake of his family or his sworn duty.
Another important quality of leadership, perhaps the most important in Tolkien's view, is mercy. Frodo has dealt mercifully with Gollum during their long journey to Mordor despite Sam's pleas to the contrary. However, when the opportunity comes, Sam also takes pity on Gollum. Now that he has worn the Ring himself, if only briefly, Sam can empathize with the miserable creature who has become "enslaved to that Ring, unable to find peace or relief ever again." This pity stays Sam's hand, even as it similarly saved Gollum's life by staying Bilbo's hand many years earlier (in 1937's The Hobbit). Sam, and all of Middle-Earth, is rewarded for this expression of pity, for when Frodo in the end resists destroying the Ring, Gollum's theft of the Ring and subsequent fall into the volcanic Crack of Doom accomplishes the Quest and destroys Sauron's power.