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[The manager]: "'He [Kurtz] is very low, very low,' he said. He considered it necessary to sigh, but neglected to be consistently sorrowful. 'We have done all we could for him - haven't we? But there is no disguising the fact, Mr. Kurtz has done more harm than good to the Company. He did not see the time was not ripe for vigorous action. Cautiously, cautiously - that's my principle. We must be cautious yet. The district is closed to us for a time. Deplorable! Upon the whole, the trade will suffer. I don't deny there is a remarkable quantity of ivory — mostly fossil. We must save it, at all events - but look how precarious the position is - and why? Because the method is unsound.'" (3.19)
Let's count the ways in which the manager is depraved: first, he misrepresents Kurtz's condition and twists his words. Second, he tries to attack Kurtz's "method" by calling it "unsound." Finally, his words are just empty—which doesn't sound so bad to us, but is evidently the tipping point for Marlow. This guy is definitely getting unfriended.
"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)
You know things are bad when you're looking to the depraved warlord for moral and mental 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)
Heading into the interior teaches Marlow that there's really no such thing as good or evil: there's only evil and slightly less evil.