If "justice" was James's highest principle, then for Lupin, not harming others was his highest principle.
Sirius had used Remus's irrational Werewolf state to attack Severus. Even if Remus escaped legal punishment, he could not escape his own conscience, though in a sense, he was, like Sirius, a "traitor to his bloodline."
Before Fenrir Greyback appeared, Werewolves were not so fond of biting and did not treat spreading "wolf venom" to as many people as possible as their purpose in life.
"Blood is what Werewolves are entitled to. Werewolves should take revenge on normal people."
A wizard had described the current state of Werewolves in the Daily Prophet as a "community of life." Within this community, the only existing subject was "the existence of the community itself," and its important mission was the continuation of the bloodline.
Although human families were also founded upon the bonds of life, this pure model of a community of life could no longer be seen in modern families. If a member of such a community wished to fulfill their individual will, what they needed to do was break free of the family's restraints.
Andromeda had also been disowned because she had refused to obey her family's wishes, marrying the Muggle-born Tonks and bearing children who were not so "pure."
Sirius had betrayed his family largely because of his mother's "tyranny." She had wanted Sirius to "obey" her, especially regarding his "standards for choosing a spouse."
What Sirius's mother found satisfactory, Sirius did not. Even when she later made a "concession," allowing him to continue living "freely" as long as he produced an heir to her liking. For the eldest son of a pure-blood family, his foremost duty was the continuation of the bloodline and the interests of the family, and the two were often