In 20,000 Leagues under the Sea, Nemo's outlook on life can reasonably be described as sympathetic to the underdog, to those suffering from oppression and persecution.
To a considerable extent, this attitude arises from Nemo's own experiences, which suggest that he himself has been the victim of persecution. Nemo believes that he is being pursued by an imperial power, which may or may not have been responsible for killing his family.
Whatever the truth of the matter, Captain Nemo is determined to exact a terrible revenge on those he feels are out to get him. It is in this desire for vengeance that we observe one of the many paradoxes concerning Nemo's character.
On the one hand, he sympathizes with the oppressed and downtrodden of the world. Yet on the other, he's a vengeful, tyrannical man whose self-isolation would appear to hint at misanthropy, an intense dislike for people. Nemo's inability to reconcile these two contradictory sides to his personality is largely responsible for his gradual mental deterioration.