The helmsman who is responsible for steering Marlow's boat is disliked by Marlow because he would act as if he was very important in front of people but he would be very "passive when left alone". Marlow considers him a fool and he demonstrates this when the boat is attacked. He is becomes very scared and drops pole in order to get a rifle and in the process is hit by a spear.He dies and is buried when Marlow throws his body into the Congo. This mirrors the burial of Kurtz, whose body is thrown in to mud hole along the Congo. The author seems to be commenting that both characters were foolish and "dropped" their real purpose in order to grab something less important. Both end up dying because what they grasped for couldn't protect them. The rifle couldn't protect the helmsman from the spear and Kurtz' power couldn't protect him from the ravages of the jungle or his own insanity.
In addition, the helmsman lacks restraint, a quality Marlow very much admires. The cannibals, oddly enough, do have restraint, for even when they exhaust their food supply of hippo meat and go hungry for some time, they manage to restrain themselves from eating the people on the boat. Earlier one of the cannibals had indicated they would eat any natives on the bank whom the boat had passed by had any of these people been killed. When they're actually hungry, however, this crew on the boat doesn't eat anybody. Marlow does, nevertheless, deposit the helmsman's body in the river so they won't be tempted to eat him.
Kurtz as well lacks restraint. He "had given in to his various lusts," according to Marlow, and sunk to utter depravity. Both he and the helmsman die, as ms-mcgregor has indicated, because of their actions.
While Marlow criticizes the helmsman in certain passages, and changes his shoes and socks after his death, it's important to note that he mourns the helmsman as well. On pg 50-51 of the Norton Critical Edition (4th edition) he says he "missed [his] late helmsman awfully" because "he had done something, he had steered; for months I had him at my back--a help--an instrument. It was a kind of partnership." He also says on page 50, "I am not prepared to affirm the fellow [meaning Kurtz] was exactly worth the life we lost [meaning the helmsman] in getting to him [Kurtz]."
This is important because he is valuing the helmsman above Kurtz. The bond between seamen is an important theme in the book, and Marlow bonded with the helmsman because of his willingness to work, coupled with his eventual dislike of Kurtz.