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"You should have seen the pilgrims stare! They had no heart to grin, or even to revile me: but I believe they thought me gone mad - with fright, maybe. I delivered a regular lecture." (2.17)
The pilgrims think their captain Marlow has gone mad with fear when he does something as mundane as giving a lecture while everyone else is freaking out from paranoia.
"The original Kurtz had been educated partly in England, and - as he was good enough to say himself — his sympathies were in the right place. His mother was half-English, his father was half-French. All Europe contributed to the making of Kurtz […]." (2.29)
The fact that "all Europe contributed to the making of Kurtz" and the subsequent fact that Kurtz went mad in the wilderness suggests that all of Europe contributes something to mankind that makes them susceptible to madness. Maybe something is wrong with the way Europe is conditioning and educating and raising its citizens. Or maybe everyone has the seeds of madness, and maybe the Africans would be just as bad in the same situation.
"There were no practical hints to interrupt the magic current of phrases, unless a kind of note at the foot of the last page, scrawled evidently much later, in an unsteady hand, may be regarded as the exposition of a method. It was very simple, and at the end of that moving appeal to every altruistic sentiment it blazed at you, luminous and terrifying, like a flash of lightning in a serene sky: 'Exterminate all the brutes!' The curious part was that he had apparently forgotten all about that valuable postscriptum, because, later on, when he in a sense came to himself, he repeatedly entreated me to take good care of 'my pamphlet'(he called it), as it was sure to have in the future a good influence upon his career." (2.29)
Here is one of the first signs of Kurtz's madness: the fact that the tone of his postscriptum differs so sharply from the rest of the manuscript. Kurtz isn't rational and idealistic anymore; he's desperate and deranged—so desperate and deranged that he apparently doesn't even remember it later, or doesn't think that it's, um, slightly problematic.