What does Kurtz in Heart of Darkness compare the scribble at the end of his paper with?

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":

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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:

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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.