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"It seemed to me I had never breathed an atmosphere so vile, and I turned mentally to Kurtz for relief - positively for relief." (3.19)
Marlow realizes that the entire environment is evil, so evil that he turns to the somewhat less corrupted Kurtz for relief.
"My hour of favour was over; I found myself lumped along with Kurtz as a partisan of methods for which the time was not ripe: I was unsound! Ah! but it was something to have at least a choice of nightmares." (3.19)
Marlow is forced to choose between two evils because there is no goodness in the interior. This tells readers just how corrupt the manager is for Marlow to side with Kurtz.
"I had turned to the wilderness really, not to Mr. Kurtz, who, I was ready to admit, was as good as buried." (3.20)
Marlow sides himself with the wilderness, but only because Kurtz is as good as dead.