Although The Strange Case of Dr. Jekyll and Mr. Hyde by Robert Louis Stevenson is best known for its protagonist, the doctor who manages to separate his good and bad natures off from one another by a mysterious potion, the narrator of the story, Gabriel John Utterson, is in some ways just as interesting a character.
Stevenson himself was raised a Calvinist, and though he abandoned his early interest in religion and became an atheist, strong religious overtones still permeate this work. In a sense, while Mr. Hyde represents sins of commission, Mr. Utterson represents sins of omission. In other words, Mr. Hyde does many evil acts, but Utterson stands by and lets evil things happen which he might have been able to prevent had he abandoned his stance of neutral, objective curiosity. In many ways, Utterson's moral failings are those which we now consider the virtues of the scientist or journalist or lawyer (his actual profession), of objective neutrality. Like Jekyll/Hyde, Utterson too is a doubled character. His professional persona's strength is his calm, nonjudgmental objectivity and ability to stand outside the events he observes; his human persona is capable of sympathy and moral discernment, and often stands by staunchly even those of whom other despair. While the protagonist Jekyll/Hyde disintegrates in trying to separate these two parts of himself, Utterson gradually increases in self-awareness and begins to integrate his two natures.
The "sin of Cain" is, as noted in the other answers, an unwillingness to get involved or take action. Unlike the "good Samaritan" who feels obliged to care for others, Cain asks "Am I my brother's keeper?" disclaiming all responsibility for other people. Thus Utterson is described as follows:
"I incline to Cain's heresy," he used to say quaintly: "I let my brother go to the devil in his own way."
By the end of the novel, we begin to understand that at times intervention is necessary. For example, if you see one of your classmates about to drive when drunk, you should take away the classmate's keys. If you let the classmate drive drunk, you are morally, if not legally, culpable if the classmate injures herself or others. Moral goodness, Stevenson is arguing, depends upon being one's bother's keeper.