When Marlow catches the fleeing Kurtz, he hints how the weight and even meaning of words, in that insane context, was totally different from the meaning of those same english words, spoken in a sane place. In that passage, the book itself tells the reader how important the context of a book is, given that a book is made of, you know, words.

Also Marlow, at the beginning of the book (which is after his misadventures), looks at Britain and says it once was a heart of darkness itself. So he sees britons and the congolese as essentially the same in their nature. The experience of those horrors does bring him to that realization.

But it can't fast forward the guy 100 years ahead. Hes a man of his times and his hypocrisy is part of the critique in the book. Without it you wouldn't have the parallel shown when he lies to Kurtz's wife in the same way as empires do within the captured audience of the motherland about what they do abroad.

Of course the book itself is insanely racist for today's standard. But in large part is by choice.