Wolf Larsen is the brutal captain of The Ghost, a seal-hunting schooner. He is a nihilist, believing in nothing but his own strength and survival; he has no regard for human life or for emotion, and treats the people around him almost as pawns to be used up and then discarded. While he is the antagonist of the book, he is not exactly evil, but instead is living according to his own moral principles, which are at odds with most civilized norms. When the crew attempts to mutiny, they believe themselves in the right because of Wolf's incredible cruelty and dispassion.
"It gives a thrill to life," he explained to me, "when life is carried in one's hand. Man is a natural gambler, and life is the biggest stake he can lay. The greater the odds, the greater the thrill. [...] He is living more royally than any man for'ard, though he does not know it. For he has what they have not -- purpose, something to do and be done, an all-absorbing end to strive to attain, the desire to kill me, the hope that he may kill me."
(London, The Sea-Wolf, gutenberg.org)
Since Wolf takes pleasure in tormenting his crew, in part because he places as little value on his own life as he does on theirs, their actions can be justified as the moral obligation to protect one's own life. While the action is, in and of itself, immoral -- in the sense that actively seeking to commit murder is immoral -- their reasons can be understood and it is easy to sympathize with them. However, the crew themselves are not exactly paragons of virtue, and the mutiny is as much for their own selfish desires as for any justifiable moral reason; in other words, it might be possible to justify the mutiny pragmatically, but it would be harder to justify it for ethical or moral reasons.