Sure, Captain Nemo may be the man behind the Nautilus and our undersea tour guide extraordinaire. But 20,000 Leagues Under the Sea isn't his book. No, 20,000 Leagues is, for better and for worse, pure Pierre Aronnax.
At the end of the novel, after he, Conseil, and Ned wash up on the coasts of Norway, Aronnax tells us he is not "revising the tale of these adventures" (2.23.3). He continues:
Not a single fact has been omitted, not the slightest detail exaggerated. It is the faithful narration of an incredible expedition. (2.23.3)
The thing is, when authors tell you that their work is totally, completely, 100% true, well… it's usually best not to believe them.
Yes, the scientist in Aronnax definitely believes he's being "faithful" to the truth. But think about it this way: Aronnax is so dazzled by Nemo, so befuddled by the Nautilus, and so frightened by his circumstances, that he doesn't fully understand Nemo's true nature until the very end of the book. Like, way after you already knew what was up, to be sure.
Aronnax wonders about Nemo's motives and origins throughout the entire book. But it's only in the final couple chapters, once the good captain has actually destroyed a ship right in front of his eyes, that Aronnax fully recognizes his host's capacity for violence.
If Verne didn't force us to learn about the characters' adventures through Aronnax's limited vision, "the message" of the book might have been very different. Aronnax's susceptibility to Nemo's charms mirrors our general human fallibility for questionably evil people's outer shininess.
Like many well-spoken, but terrifyingly violent military leaders throughout history, Nemo is able to keep Aronnax under his thumb for most of this book.