For someone who wrote a lot of big words, Conrad seems to have some serious doubts about the power of language. In Heart of Darkness, words are always trying and failing to live up to their big, impressive goal: ensuring that two people can understand each other. For Kurtz, language is a way to justify white man's superiority over the Africans. For Marlow, language represents a way out of madness by establishing a connection with other humans. For the Africans—well, who knows? To Marlow, their words aren't even language. We might say the same about you, Mr. Conrad.
Linguistic expression—through either speech or text—represents one way out of madness, but they also represent a way into madness. Kurtz's handle on language helps make him go crazy.
In Heart of Darkness, problems with words give us a clue to character defects.