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"The fact is I was completely unnerved by a sheer blank fright, pure abstract terror, unconnected with any distinct shape of physical danger. What made this emotion so overpowering was - how shall I define it? — the moral shock I received, as if something altogether monstrous, intolerable to thought and odious to the soul, had been thrust upon me unexpectedly." (3.24)
The "moral shock" that Marlow feels upon realizing Kurtz is gone is probably a severing of the connection he felt for Kurtz for, after all, the two have had remarkably similar experiences. The fact that Marlow just recently chose Kurtz over the manager and the Company also contributes to his reaction.
"I did not betray Mr. Kurtz - it was ordered I should never betray him—it was written I should be loyal to the nightmare of my choice." (3.25)
Marlow considers Kurtz the "nightmare of [his] choice." He knows that Kurtz is corrupt, but he adheres loyally to him. Thus Marlow sides with a lesser evil than the Company and the manager.
"A black figure stood up, strode on long black legs, waving long black arms, across the glow. It had horns - antelope horns, I think - on its head. Some sorcerer, some witch-man, no doubt: it looked fiendlike enough." (3.28)
Readers implicitly regard this figure with fear and revulsion because it is steeped in devil imagery – blackness, horns, and a silhouette against a fire.