What good action movie (or sci fi book) doesn't have a character who lives for revenge? "Do you think I am unaware there are suffering beings and oppressed races on this planet," Nemo asks Aronnax in 20,000 Leagues Under the Sea, "wretches to be helped and victims to be avenged?" (2.8.75). The thing is, whether or not Nemo knows about these "wretches"—and he certainly seems to—he's not killing people just to right mankind's wrongs. He has a personal score to settle, one that he refuses to fully explain. And this confusion of personal motives and idealist, nation-state kinda-motives makes it hard for us to know how "good" or "evil" Nemo really is. Is Nemo using this whole business about "oppressed races" just to justify his personal vendettas?
Nemo's thirst for revenge is entirely justified by the tragedies he has endured.
Nemo's quest for vengeance is aimless and endless; his only missions are to survive and to allow the oppressed peoples of the world (whomever they may be) to continue fighting.