We here at Shmoop are pretty suspicious of anyone who tries to enforce martial law. In 20,000 Leagues Under the Sea, Aronnax calls Nemo a "terrible lawgiver" (2.21.91). But we never get a sense of what those laws are. All we know is that Nemo's got a chip on his shoulder and a heck of a weapon—and this leads to some pretty questionable scenarios. Aronnax is forced to wonder whether Nemo's attacks can possibly be justified. And then there's that whole "You guys can never, ever leave my submarine" bit. Apparently, though Nemo may speak out against tyrants and despots, there's more than a little of his sworn enemies inside of him.
No matter how we view them, Nemo's actions are neither just nor unjust at core. By separating himself from humanity, Nemo has separated himself from our normative ideas of morality and justice.
Though he claims to have removed himself from all regular notions of justice, Nemo's actions contradict his words. When he deems one group of people the "oppressed" and another the "oppressors," he is adopting the moral stances of society at large.