As Chillingworth becomes more and more obsessed with discovering the person Hester committed adultery with and punishing him, he becomes a more and more evil and twisted character. He suspects Dimmesdale, for he can tell something is weighing down his conscience, and so he studies him as if Dimmesdale is one of his medicinal plants specimens:
delving among his principles, prying into his recollections, and probing everything with a cautious touch . . .
Chillingworth becomes Dimmesdale's physician, and during his obsessive concentration on the ailing man, he notices a letter "A" marked on his chest. This confirms for him that Dimmesdale was Hester's lover and is Pearl's father.
Chillingworth's jealousy, bitterness, and resentment turn him hateful. We learn that he is “haunted either by Satan himself or Satan’s emissary.” He is glad when Dimmesdale is in pain. He thwarts Hester's effort to leave with Dimmesdale, but in the end, Dimmesdale inadvertently steals Chillingworth's thunder by confessing in public to his sin.
If the "scarlet letter" of adultery becomes a vehicle that allows Hester to grow and mature spiritually, so that she can become a help and joy to her community, knowledge of his wife's adultery eats away at Chillingworth's soul and turns him to evil. Hawthorne thus suggests that it is not the sin itself but how we respond to it that matters most.