The Question and Answer section for Dr. Jekyll and Mr. Hyde is a great resource to ask questions, find answers, and discuss the novel.
In the story of Cain and Abel, Cain murders his brother. In the above line, Utterson is citing his belief that one should stay out of other people's business.
Jekyll has become unsure of himself, sickly, faint, and desperate. He is not the self-assured, smooth faced man we met at the dinner party in the third chapter.
You can check this out in chapter 8 summary below:
https://www.gradesaver.com/dr-jekyll-and-mr-hyde/study-guide/summary-chapters-7-8
