Marlow's obsession with Kurtz is due to the fact that he compares himself to Kurtz, who is a charismatic, respectable character. However, Kurtz is weaker than Marlow in the way that Kurtz basically goes crazy because he is power hungry, while Marlow realizes how dangerous and unwise that could be. It is evident that Marlow is greater and stronger than Kurtz because Kurtz dies of an illness, but when Marlow gets sick, he doesn't die.
Marlow's journey consists of observing Kurtz's activities. As a metaphor, Marlow is peering over the edge of a chasm with Kurtz at the bottom. Marlow is close enough to see Kurtz, but Marlow does not fall in himself. In the end, Marlow has learned more about himself and he knows what mistakes Kurtz has made himself. He has also learned what kind of a person he strives to be, based upon Kurtz's actions when he was alive.
Marlow mentions that Kurtz was so great because he "had something to say. He said it". Marlow is basically the opposite because he is more of a quiet thinker. He doesn't always speak what is on his mind; he is less action, and more calculation. Because Kurtz dies and Marlow doesn't, the author could be suggesting that people that try to obtain too much power will always lose. The novel also suggests a theme of karma. Kurtz seems to realize what mistakes he has made based upon his dying words,"The horror, the horror". Of course, many different interpretation could be made based upon these vague words.
