How is Kurtz an example of spiritual degeneration?
In Heart of Darkness, Mr. Kurtz is a symbol of how colonialism changes the colonizers as well as the colonized. It is an illustration of the law of compensation which Ralph Waldo Emerson deals with in what is one of his best-known and most profound essays.
Perhaps Kurtz is so often called Mister in order to remind everyone, including the reader, that he was once a civilized European gentleman. The battle between civilization and animal savagery goes on inside every one of us. We all have only a thin veneer of civilization.
Shortly before he dies, Kurtz cries, "The horror! The horror!" Perhaps what horrifies him is that he sees the change that is taking place within himself. He is ruling his little empire by force and terror. He no longer has even a condescending, colonialist sort of good will toward the natives. Kurtz, and the Belgian colonizers operate with sheer brutal exploitation motivated by greed, lust and sadism.
The dark heart of Africa in the depths of the Belgian Congo is symbolic of the darkness in every human heart. Marlow doesn't say as much, but the reader draws more from Marlow's story that the narrator is aware of imparting.