I saw the enigmatic individual as essentially pitiless and cruel, as he was forced to be. I felt him as being beyond the pale of humanity, insensible to feelings of pity, the remorseless enemy of his fellow beings, against whom he must have sworn an undying hatred. (1.9.63)
The key word here seems to be forced. Aronnax suggests that Nemo can't help but hate whomever or whatever caused his pain.
Part 1 Chapter 14
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 kind of canonizing Nemo here. This quote points to Aronnax's strongly ambivalent relationship with the dude. On the one hand, he's suspicious of the captain from the start. On the other hand, he's so impressed with the captain's inventions and daily life that he reveres Nemo in a scary, Celebrity Worship Syndrome way.
Part 2 Chapter 12
Captain Nemo joined us.
"Well, Master Land?" he enquired.
"Well, monsieur," replied the Canadian, whose enthusiasm had diminished; "it was a terrible sight indeed. But I am not a butcher, I am a hunter, and this was just butchers' work."
"It was a massacre of evil animals," said the captain, "and the Nautilus is not a butcher's knife."
"I prefer my harpoon."
"To each his weapon," replied Nemo, staring at Ned. (2.12.117-122)
Nemo is more than a misanthrope. He evidently hates some other "evil" animals as well. This quote shows us just how far Nemo takes his division of the world into the "oppressors" and the "oppressed"; he's even willing, quite crazily, to apply this dichotomy to whales.
Part 2 Chapter 18
Captain Nemo, red with blood, motionless near the searchlight, examined the sea which had swallowed up one of his companions, as large tears flowed from his eyes. (2.18.109)
Here we see true compassion on Nemo's part. Who is Nemo most sympathetic to and why?
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's own hatred for humanity, Aronnax suggests, is capable of fostering hate in others—even in our level-headed harpooner, Ned.
Part 2 Chapter 20
The enterprise had succeeded, and in its first telegram, young America sent old Europe wise words which are so rarely understood: "Glory to God in the highest, and on earth peace, good will towards men." (2.20.31)
It's sad to think that a message of such hope and kindness is "so rarely understood." You have to wonder if Nemo is still capable of understanding those words.
Part 2 Chapter 21
I turned to Captain Nemo. That terrible lawgiver, that archangel of hate, was watching still. When everything was finished, Captain Nemo headed for the door of his room, opened it, and went in. My eyes followed him.
On the far well, below the pictures of his heroes, I could see the portrait of a woman, still young, with two small children. Captain Nemo looked at them for a few moments, stretched out his arms to them, and then knelt down sobbing. (2.21.91-2)
Nemo's hatred is so intense, it transforms him into an "archangel of hate"—it places him above (or below, depending on your perspective) humanity. This description of Nemo isn't the only time Aronnax thinks of his Captain as larger-than-life, though. Is Nemo more myth or man?
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)
If Nemo's hatred is "monstrous," does that make him a monster? If it's "sublime," does that make him sublime? What Big Life Questions is Aronnax really struggling with here?
"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)
Here, Nemo seems to have lost sight of the specificity of the actual warship that's attacking him. It's not really a ship anymore, it's a symbolfor all of the atrocities that have been perpetrated against him. This is yet another sign that our man Nemo's gone off the deep end (haha) of hatred, straight down into Crazy Town.