Heart of Darkness can be read for free on various websites, including eNotes and gutenberg.org. Here is a brief summary.
The unnamed narrator is traveling on a small boat with four other men. One man, Marlow, begins to talk philosophically about his experiences sailing the rivers of the Congo, and about a mission to find a missing ivory hunter, Kurtz. Marlow is surprised at what he sees as the lack of civilization in the jungle, and as he ventures deeper down the river he becomes more and more uneasy. He sees that some of the stations are in disrepair, and that the Europeans who live in the jungle are strange. Eventually, after several setbacks and suffering a native attack, Marlow finds Kurtz deathly ill, and discovers that Kurtz has been using his power and influence to become a dictator over his own men and over the natives. While traveling back upriver, Marlow listens to Kurtz raving in his delirium, and realizes that his own experience was not unique.
Since I had peeped over the edge myself, I understand better the meaning of his stare, that could not see the flame of the candle, but was wide enough to embrace the whole universe, piercing enough to penetrate all the hearts that beat in the darkness.
(Conrad, Heart of Darkness, gutenberg.org)
The difference between Kurtz and Marlow is that Kurtz embraced the removal of societal restraint and enjoyed using his power to control and kill. Kurtz finally dies and Marlow settles his affairs on shore; Marlow's last action is to meet the woman whom Kurtz intended to marry, and reassure her -- a sympathetic white lie -- that Kurtz was not truly mad, and that he loved her to the end. The unnamed narrator looks out over the river, and sees some of the darkness that Marlow has described.