I have deleted your question on Kurtz because only one question per posting is allowed. But perhaps the responses you get will help you to understand how the darkness affected him.
The external darkness is the Africans, who are dark-skinned people, and the darkness of the jungle, where light seldom penetrates. The internal darkness is the darkness of colonialism, its arrogance, greed, and willful ignorance of the cultures and peoples it exploits, which stains the hearts and souls of the colonial nations, in this case the British, but also true of colonialists in general. While the colonialists regarded the Africans and the country as dark, Conrad shows clearly, in nearly every scene in the book, who it is who is really "dark."