To throw out another possibility... Many have suggested that the central question of the novella is exactly what lies at the core of the human soul. Are we all just waiting for the conditions and a context just right to trigger our basic evil instincts? Clearly we see that Kurtz has descended into his own world of evil and madness. In some ways, Marlow seems to be following a similar path, although his fate seems far less determined. When I read Heart of Darkness I can't help but wonder what horrific acts I might be capable of if placed in a similar situation. Although I would certainly hope to maintain my morals and my beliefs, can I ever be completely assured of that fact?
I am sure you will get several ideas on this one, but one central question that comes to mind for me as I consider the themes of good, evil, truth and morality is, "Can man change?" More specifically, I wonder if man can go from bad to good. It seems easy to move in the other direction throughout history, but this direction is much, much harder to come by.
This is complicated by the unreliability of the narrator. If the narrator weren't tainted and if we could trust each perception, coming to a better conclusion on your question would be easy. Perhaps though, that is part of what contributes to the question of change for the better.