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Toward the end of Francis Coppola's "Apocalypse Now," the camera casually, almost randomly, roams across a disheveled hut, passing a small number of scattered books lying in such a way as to suggest recent usage. One of these books is Sir James Frazer's "The Golden Bough." It is no accident. Indeed, this book holds the key to understanding the conclusion of the film that has baffled—and annoyed—most critics and will very likely be unsettling a number of moviegoers now that "Apocalypse Now" is entering general distribution….
If, as most critics have done, we look to Joseph Conrad's "Heart of Darkness" for the answer, we will be … disappointed. True, the film does rely upon Conrad's novella for its characters and plot structure, but still it does not explain Mr. Coppola's ending: In the book, Marlow, a young ship's captain, is hired by a rubber firm to guide a boatload of greedy Europeans (Conrad calls them "pilgrims") up the Congo River to the conpany's central outpost, a camp run by an agent named Kurtz. Marlow discovers that this Kurtz has undergone a transformation during his many years of isolation, that the man of culture and ideas had become something primitive and barbaric, a veritable god among the natives….
But Marlow has not come to kill Kurtz; on the contrary, he does all in his power to retrieve the sickly agent back to England. In the Conrad novel, Kurtz's death is, in a sense, anticlimactic, resulting at least ostensibly from fever. Obviously "Heart of Darkness" has taken Mr. Coppola just so far; to follow the film to its conclusion we must turn to "The Golden Bough."…
[The evidence becomes strong] when in the following paragraph Frazer writes: "The mystic kings of Fire and Water in Cambodia are not allowed to die a natural death. Hence, when one of them is seriously ill and the elders think that he cannot recover, they stab him to death," (emphasis mine).
It is from Frazer and not Conrad that Mr. Coppola has borrowed the location, Cambodia, and the mode of death, stabbing. Perhaps more subtle, but no less telling, is the notion of Mr. Coppola's Kurtz as king of "Fire and Water"—Frazer's symbols which the director exploits and builds upon from the film's very first frame. Fire plays almost no role at all in "Heart of Darkness," whereas in "Apocalypse Now" it operates on several levels—firepower, napalm, burning villages.
https://www.enotes.com/topics/francis-ford-coppola/criticism/coppola-francis-ford-vol-16/john-tessitore