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"And then that imbecile crowd down on the deck started their little fun, and I could see nothing more for smoke." (3.35)
The "imbecile crowd" of white pilgrims are evil because they want to shoot and kill the native Africans simply for a "little fun."
"But both the diabolic love and the unearthly hate of the mysteries it had penetrated fought for the possession of that soul satiated with primitive emotions, avid of lying fame, of sham distinction, of all the appearances of success and power." (3.37)
Kurtz is ruled by evil, but even in its reign, it will not leave him in peace. Two evils fight for Kurtz’s soul and both fill his mind with greedy, if unrealistic, thoughts.
"His [Kurtz’s] was an impenetrable darkness. I looked at him as you peer down at a man who is lying at the bottom of a precipice where the sun never shines." (3.40)
Kurtz is so claimed by evil that, metaphorically, light cannot touch him anymore.