The report you refer to comes in the middle section of the book. Marlow discovers that the International Society for the Suppression of Savage Customs had entrusted him with the making of a report, "for its future guidance." Marlow describes how eloquent the report is - "it was a beautiful piece of writing":
From that point he soared and took me with him. The peroration was magnificent, though difficult to remember, you know. It gave me the notion of an exotic Immensity rules by an august Benevolence. It made me tingle with enthusiasm. This was the unbounded power of eloquence - of words - of burning noble words.
Clearly this report allows Marlow to see Kurtz's rhetoric powers of inspiration and communicates what an amazing orator Kurtz was. Although it was rousing, Marlow says that there were no statements of practical application until the end comment:
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 the brutes!"
Here we can see then that this final scrawl is compared by use of a simile to an unexpected flash of lightning in a calm, peaceful sky. Note how Conrad builds up the comparison, emphasising the altruistic nature of the report, which stands in harsh contrast to the final statement. Remember, this report gives us an insight into Kurtz's mind before we have even met him, and the last postscript authorising genocide seems to hint at the kind of mental degradation and moral collapse that Kurtz has undergone.