Would I ever know to what nation this strange man belonged, that boasted of belonging to none? Who had produced the hatred he had sworn for the whole of humanity, the hatred which might perhaps seek a terrible vengeance? Was he one of those unrecognized scientists, one of those geniuses 'who had been hurt' to use Conseil's expression, a modern Galileo; or he was he one of those scientists […] whose career was ruined by a political revolution? (1.14.24)
Aronnax is awfully quick to guess that Nemo is out for revenge. Why do you think that is?
Part 1 Chapter 22
I could easily have shot this native, within close range, but I believed it better to wait for really hostile behaviour. When dealing with savages, it is better for the Europeans to riposte, rather than attack first. (1.22.39)
We think this is an awfully condescending way for Aronnax to treat the "natives." Aronnax seems to care less about killing a "savage" than he does about keeping up appearances—he wants to act like a good European should. Aronnax's investment in the social mores of his home continent speaks to his generally overstated nationalism. He's less afraid of being eating by cannibals than he is of appearing foolish.
Part 2 Chapter 8
"Never be of any use!" he replied animatedly. "What makes you believe, monsieur, that these riches must be considered wasted if I collect them? Do you think that it is for my own benefit that I take the trouble to gather these treasures? Who told you that I do not put them to good use? Do you think I am unaware there are suffering beings and oppressed races on this planet, wretches to be helped and victims to be avenged? Don't you understand?" (2.8.75)
Nemo seems to suggest that his actions are selfless; he's not looking to avenge his loved ones, but to help out some nameless "victims." Who exactly those victims are, we don't know. We do know that it's hard to figure out whether Nemo's a good-but-misunderstood type, or a vengeful monster to the core.
Part 2 Chapter 19
Even on the occasions when our hearts have gone out to you, moved by some of your pain or your acts of genius or courage, we have had to hide all signs of the sympathy that comes from the sight of what is fine and good, whether displayed by friend or enemy. Well, it is this feeling that we are foreign to everything that concerns you which makes our position untenable. An impossible one even for me, but doubly impossible for Ned. Every human being, by the very fact of being human, is worthy of respect. Have you ever asked yourself what plans of vengeance could be engendered by the love of freedom and hatred of slavery in a nature like the Canadian's, what he could think or do? (2.19.58)
Nemo needs to be reminded that he's not the only once capable of seeking revenge. Arronax thinks that Nemo's imprisonment of Ned will lead Ned to go after him. And he's probably not wrong; the French-Canadian hunter we know and love may have simple desires, but he'll do just about anything to satisfy them. Ned's a staunch individualist, and he believes in his personal liberties.
Part 2 Chapter 20
"Seventy-four years ago to the day, on this same spot of 47 degrees 24' N, 17 degrees 28' W, this ship lost two of its three masts in a heroic battle; it had taken on water and a third of its crew were out of action. It preferred to scuttle itself with its 356 crew rather than surrender. Nailing its flag to the poop, it disappeared under the waves with the cry 'Long live the Republic!'"
"The Vengeur!" I exclaimed.
"Yes, monsieur. The Vengeur! A fine name!" said Captain Nemo, crossing his arms. (2.20.51-3)
Nemo gives us some clues to his identity here. Even if he isn't French, he sympathizes with the cause of the French Republicans. Maybe, like them, he once fought against a tyrannical monarchy.
Part 2 Chapter 21
Perhaps I would never know who he was, where he came from, where he was going, but I could see the man more and more distinctly from the scientist. It was not common misanthropy that had enclosed Captain Nemo and his companions in the flanks of the Nautilus, but a monstrous or sublime hatred that time could not diminish.
Was this hatred still seeking revenge? The future would soon show me. (2.21.1-2)
Aronnax seems to think that Nemo and his crew need some kind of outlet for their hatred. I guess he doesn't believe that whole "time heals all wounds" adage. When it comes to Nemo and his exile buddies, maybe the saying should be something more like, "only vengeance will do."
"I am the law, I am the justice!" he said. "I am the oppressed, and they are the oppressor! It is because of them that everything I loved, cherished, venerated—county, wife, children, parents—perished as I watched! Everything I hate is there! Keep quiet!" (2.21.59)
Even as Nemo is attacking a specific ship, we can't tell if that ship is actually associated with some particular enemy or enemy nation. Maybe this one warship just symbolizes everything Nemo hates...
Part 2 Chapter 23
But what became of the Nautilus? Did it resist the embrace of the Maelstrom? Is Captain Nemo still alive? Is he continuing his terrifying reprisals under the ocean, or did he stop at his last massacre? Will the waves one day wash up the manuscript containing the entire story of his life? Will I finally discover his name? Will the nationality of the vessel sunk tell us Captain Nemo's own nationality? (2.23.5-6)
After watching Nemo destroy the mysterious warship, Aronnax still isn't sure Nemo's gotten the release he needs. At this point, he's realized Nemo's misanthropy runs to some scary, scary depths.