There's a word for Nemo's particular brand of hatin': misanthropy. That's a fancy term for "a dislike of mankind." In 20,000 Leagues Under the Sea, we never really know who has caused Nemo's pain. So, the ever-lurking, ambiguous nature of his enemies makes us feel like everyone could is his enemy. Nemo's hatred of mankind doesn't prevent him from liking certain individuals, however. He gets along with his crew just fine, he works with the Greek diver, and he saves the poor Indian pearl diver. Plus, despite their differences, he seems to like Aronnax. So Nemo's sort of a mixed bag of fiery hate and not-so-fiery like, after all.
Hatred defines Nemo and powers his intellect; without it, he would be not only no one, but nothing.
Though he may seem hateful on the surface, Nemo's commitment to the oppressed peoples of the world suggests that his feelings are much more complex.