We can’t decide if Marlow is a foil to Kurtz, or if Kurtz is a foil to Marlow. Either way, the two men reflect certain disturbing aspects of each other. It seems that much of the novel depicts Marlow’s slow decay and transformation into the corrupted Kurtz, but stops one vital step short. Like Kurtz, Marlow shows a certain amount of respect to the native Africans – admiring their physical strength and sympathizing with their plight, especially at the Outer Station.
However, this is not true respect as we 21st century Americans would deem it. Marlow, like Kurtz, does not see the native Africans as the white man’s equal and he buys in, to some degree, to all the bombastic rhetoric of "civilizing" them. This shows such a disturbing disdain for their cultural identity and intelligence that it begins to echo Kurtz’s eloquent diary where he urges, at one point (the crazed postscript point), for the Company to eliminate all the savages. Marlow shares the same fascination with language that Kurtz nurtures, understanding that one’s articulation in language can be the difference between plunging into madness and reconciling oneself with the true evil of the situation.
The helmsman isn’t really a foil to Kurtz. But their deaths definitely do share some odd parallels. Both men die in the same place on board Marlow’s steamship. Marlow declares that the helmsman would have been fine had he just "left that shutter alone," while Kurtz stares through the very same shutter unit he makes a point of telling Marlow to close it. Marlow throws his shoes overboard just after the death of the helmsman, while he gives away a pair of shoes shortly before the death of Kurtz. These are small and insignificant on their own, but they start to build us a picture for the more important stuff. Here comes the more important stuff.
Marlow says of his dead helmsman that "he had no restraint, no restraint – just like Kurtz – a tree swayed by the wind." Well, that’s rather explicit. But if you missed it the first time, Marlow later says that Kurtz is – you got it, yes, "a tree swayed by the wind," which harkens right back to this passage. In fact, the helmsman’s death is the point at which Marlow breaks from his story and alludes to several events that are yet to come in his narrative, including his meeting Kurtz, the man’s death, and his (Marlow’s) visit to the Intended. It is interesting that this point, the helmsman’s death, is where we get this odd and actually sort of confusing break.
But, like any good foil, the death of the helmsman isn’t identical to the death of Kurtz. The most important difference is what they say (or do not say) before they die. Kurtz, of course, utters the famous "the horror" line. And the helmsman? Marlow tells us that "it looked as though he would put to us some question in an understandable language; but he died without uttering a sound." Hmm. That doesn’t give us much to work with. However, he does manage to frown noticeably before he expires, with what we are told is a "menacing expression." This sounds like foreshadowing to the message Kurtz will deliver, slightly more vocally, towards the end of the novel.
There’s one more interesting thing to pay attention to here. When Marlow throws the body of the helmsman overboard, he notes that he is "heavy, heavy; heavier than any man on earth." Yet, when he carries the sickly Kurtz back from the jungle, this guy is "not much heavier than a child." Or so he thinks at the time. Marlow ends up physically exhausted from his journey. Hmm. We see a great weight in his tossing the helmsman overboard; he shared a kinship with this man, he says, and we see that reflected in the burden of disposing of his body. But his dealing with Kurtz is a whole ‘nother weight altogether. Although the man is emaciated and thin, Marlow feels the weight of the world on his shoulders when he carries him. Kurtz isn’t just a man; he’s man’s corruption, man’s depravity, man’s greed and destruction. And that’s a lot to carry out of the jungle.