Hawthorne entitles Chapter 9 "The Leech" because he explains throughout the chapter what Roger Chillingworth (formerly Roger Prynne, Hester's husband) is doing to torture the man who committed adultery with Hester.
In regards to the tone of the chapter, Hawthorne's attitude throughout much of it is foreboding. Readers, through dramatic irony, already know what Chillingworth's plan is; so to discover that he is living with and advising Dimmesdale is troubling to say the least. At the end of the chapter, many church members have come to view the Dimmesdale-Chillingworth relationship as a struggle between good and evil for Dimmesdale's immortal soul. In their naivety, they with "unshaken hope," could not doubt "on which side the victory would turn."
They assume that Dimmesdale is on the side of righteousness, but Hawthorne warns and foreshadows that Dimmesdale's victory is far from secure.
The theme of the chapter contributes to one of the novel's main subjects--appearance versus reality. At the beginning of the chapter, Hawthorne writes about Chillingworth,
This learned stranger was exemplary as regarded at least the outward forms of a religious life.
The stress is on the word "outward" because Chillingworth like many of the townspeople is able to portray a righteous facade in order to accomplish his goals. Throughout the chapter, Hawthorne notes how Chillingworth worms his way farther into Dimmesdale's life and thoughts before anyone begins to take note that perhaps his interior is not so charitable.