We should note that Kurtz's depiction in the film isn't so much a function of trying to illustrate a set of circumstances unique to Vietnam. Instead it is pulled straight from the source material for the film. That being Conrad's Heart of Darkness a novella which is based in part on Conrad's experience as a boat captain in the Belgian Congo and set in the same during the height of King Leopold's sacking of the region.

In that Kurtz is the manager of the "Inner Station" which produced the most volume of ivory and other goods, but was also envied greatly, and eventually drops out of contact amid rumors of his going native, living like a tribal God, falling ill, and a slowing in the flow of ivory. To that end Marlow is sent up river with a small party to seek him out and return him down river to rest or face judgement as needs be.

When Marlow reaches the Inner Station he finds that Kurtz is on his deathbed, but has set himself up as a native leader as well and launched a reign of terror to subjugate nearby tribes and in general carried on setting up a cult of personality and a real cult. He then dies on the trip down river.

Much of the famous imagery from the movie is taken almost verbatim from the work, the natives silent aboard canoes, the ill Kurtz, even the journalist that the boat crew meets at the camp is based on the Russian "Harlequin" from the tale.

In effect Apocalypse Now isn't really a Vietnam Movie, though it is a very good one too, it is simply the retelling of African Colonialism with set dressing more familiar to Americans.

We can compare a few quotes from the book to the movie then and I hope it does clear up that it absolutely is fundamentally the same story. That being an examination of what happens when the darkness within man is let loose by removing all checks from it, by placing him in the literal state of nature.

Book

But his soul was mad. Being alone in the wilderness, it had looked within itself and, by heavens I tell you, it had gone mad.”

Film

I watched a snail crawl along the edge of a straight razor. That's my dream; that's my nightmare. Crawling, slithering, along the edge of a straight razor... and surviving.

And the book's depiction of Kurtz's death.

“I think the knowledge came to him at last — only at the very last. But the wilderness had found him out early, and had taken on him a terrible vengeance for the fantastic invasion. I think it had whispered to him things about himself which he did not know, things of which he had no conception till he took counsel with this great solitude — and the whisper had proved irresistibly fascinating.

Anything approaching the change that came over his features I have never seen before, and hope never to see again. Oh, I wasn’t touched. I was fascinated. It was as though a veil had been rent. I saw on that ivory face the expression of somber pride, of ruthless power, of craven terror — of an intense and hopeless despair. Did he live his life again in every detail of desire, temptation, and surrender during that supreme moment of complete knowledge? He cried in a whisper at some image, at some vision, — he cried out twice, a cry that was no more than a breath — ‘The horror! The horror!”

Kurtz in the film describing his dawning upon his new fundamental truths.

I've seen horrors... horrors that you've seen. But you have no right to call me a murderer. You have a right to kill me. You have a right to do that... but you have no right to judge me. It's impossible for words to describe what is necessary to those who do not know what horror means. Horror... Horror has a face... and you must make a friend of horror. Horror and moral terror are your friends. If they are not, then they are enemies to be feared. They are truly enemies! I remember when I was with Special Forces... seems a thousand centuries ago. We went into a camp to inoculate some children. We left the camp after we had inoculated the children for polio, and this old man came running after us and he was crying. He couldn't see. We went back there, and they had come and hacked off every inoculated arm. There they were in a pile. A pile of little arms. And I remember... I... I... I cried, I wept like some grandmother. I wanted to tear my teeth out; I didn't know what I wanted to do! And I want to remember it. I never want to forget it... I never want to forget. And then I realized... like I was shot... like I was shot with a diamond... a diamond bullet right through my forehead. And I thought, my God... the genius of that! The genius! The will to do that! Perfect, genuine, complete, crystalline, pure. And then I realized they were stronger than we, because they could stand that these were not monsters, these were men... trained cadres. These men who fought with their hearts, who had families, who had children, who were filled with love... but they had the strength... the strength... to do that. If I had ten divisions of those men, our troubles here would be over very quickly. You have to have men who are moral... and at the same time who are able to utilize their primordial instincts to kill without feeling... without passion... without judgment... without judgment! Because it's judgment that defeats us.

And Marlow's thoughts as they travel up The River, easy to see how it fit so well to SE Asia.

“Going up that river was like travelling back to the earliest beginnings of the world, when vegetation rioted on the earth and the big trees were kings. An empty stream, a great silence, an impenetrable forest. The air was warm, thick, heavy, sluggish. There was no joy in the brilliance of sunshine. The long stretches of the waterway ran on, deserted, into the gloom of overshadowed distances. On silvery sandbanks hippos and alligators sunned themselves side by side. The broadening waters flowed through a mob of wooded islands; you lost your way on that river as you would in a desert, and butted all day long against shoals, trying to find the channel, till you thought yourself bewitched and cut off forever from everything you had known once -somewhere- far away in another existence perhaps. There were moments when one's past came back to one, as it will sometimes when you have not a moment to spare to yourself; but it came in the shape of an unrestful and noisy dream, remembered with wonder amongst the overwhelming realities of this strange world of plants, and water, and silence. And this stillness of life did not in the least resemble a peace. It was the stillness of an implacable force brooding over an inscrutable intention. It looked at you with a vengeful aspect.”