I must admit, I do feel that the film of Apocalypse Now does seem to present the universal themes of Heart of Darkness in an incredible and unforgettable way. One of the sections you might like to consider is when Willard reaches the outpost of Kurtz and sees the kind of strategies that Kurtz has employed to maintain control of his territory. Dead bodies abound, and they include men that have obviously been tortured and hung. This of course relates to the way in which the novel presents the darkness of humanity and how, if we are in a place where there are no restricting forces such as systems of law and order, even the most brilliant of us are capable of showing how truly horrific we are as humans as our base and primeval nature overpowers us and comes to dominate us. This is shown clearly by the various "excesses" of Kurtz in the film and in the book. Consider what Marlow says about Kurtz that supports this:
I was within a hair’s-breadth of the last opportunity for pronouncement, and I found with humiliation that probably I would have nothing to say. This is the reason why I affirm that Kurtz was a remarkable man. He had something to say. He said it. . . . He had summed up—he had judged. ‘The horror!’ He was a remarkable man.
Kurtz was, by Marlow's account, a "remarkable man," and he contrasts himself with Kurtz finding himself wanting. Yet one of the reasons why Kurtz is a remarkable man is because he was able to take a look at himself and pronounce a judgement on what he had done and the moral depths into which he had fallen. Remove a man from civilisation, both the novel and the film suggest, and you reveal that you do not have a man at all, but a base and violent savage.