I agree with the ideas above suggesting that compassion, sympathy and empathy are each affirmed in this novel. At the opening of the novel, we are given a scene of quiet understanding which is somewhat connected to the scene of Kurtz death aboard ship. People, in these scenes, are literally "in it together", on the same boat, listening to one another and trying to understand.
The fact that Marlow attempts to really and truly understand a man like Kurtz (so far gone into madness) suggests that empathy is alive in Marlow to an impressive degree.
Conrad clearly defines evil through Kurtz and the colonial system that produced and protected him, but he affirms man's capacity for goodness through Marlow. It is through Marlow's reactions to Kurtz and colonialism that Conrad achieves this. Marlow is revolted by what he sees and experiences during his journey and at its conclusion. He finds Kurtz to be morally repugnant, and he is appalled by the greed, cruelty, and exploitation of colonialism. He brings Kurtz out of the jungle and takes him aboard his ship. When Kurtz tries to escape to return to his life of moral degradation in the jungle, Marlow drags him back.
When Kurtz is dying aboard ship, however, Marlow treats him with compassion, attending to his needs. Marlow has seen the worst kinds of inhumanity, but it does not destroy his own humanity. After Kurtz dies, Marlow goes to see Kurtz' fiancee in Brussels, a woman who had loved him dearly. He is faced with a moral choice. He can tell her the truth about Kurtz, or he can lie, allowing her to live with her memories and illusions. Even though Marlow is an honest man who abhors lying, he chooses the lie. He will not destroy the rest of her life with the truth. He chooses to be kind. Telling the lie violates his conscience, but inflicting pain is a worse sin for Marlow. It would violate his own humanity. Kurtz shows man's capacity to do evil, but through Marlow, Conrad affirms that the human spirit is also capable of compassion and unselfish goodness.
I agree with post #3 about how the notion of retelling stories is an aspect that is affirmed in the novel. One of the themes I take away from it is the idea that as bad and horrific as human beings can be, there is a quality of learned appreciation for hearing the narratives of others. This helps to expand our moral imagination, making us understand things that can, hopefully, allow us to do things differently and better. The modern setting is predicated upon understanding multiple and varied conceptions of the good, and certainly the way in which Conrad's novel is told might reaffirm this idea.
Given the general darkness of Conrad's classic work, it's easy to assume that nothing is affirmed. However, several things are. First, consider how often people throughout the novel attempt to follow their duties. In strange and even twisted circumstances, they persevere. Second, people follow their callings—even their destinies. It can't be easy to be Kurtz, or Marlow, but they go on. This affirms a kind of faith in fate, a willingness to be greater than one is. Third, consider the fact that the story is told. This suggests a faith in the positive power of stories, language, and learning.