Chillingworth has devoted his life to seeking revenge against Arthur Dimmesdale. In doing so, he has become as evil as his desire to harm another.
In a word, old Roger Chillingworth was a striking evidence of man's faculty of transforming himself into a devil, if he will only, for a reasonable space of time, undertake a devil's office. This unhappy person had effected such a transformation by devoting himself, for seven years, to the constant analysis of a heart full of torture, and deriving his enjoyment thence, and adding fuel to those fiery tortures which he analyzed and gloated over.
Chillingworth is obsessed with the idea of Dimmesdale's suffering; his life is devoted to it. There is nothing about which he is more passionate; he is excited and invigorated by the though of Dimmesdale enduring the torture inflicted upon him. Chillingworth has even become so transformed by his devotion to revenge that, upon reflection on the person he has become, he refers to himself as "A fiend!" He is no longer the
earnest, studious, thoughtful, quiet...thoughtful for others, craving little for himself--kind, true, just and of constant if not warm affection
man he once was.