In Chapter XIV, a chapter in which Chillingworth and his wife, Hester Prynne, discuss the Reverend Dimmesdale, Hester's punishment, and Chillingworth's machinations, the narrator says, "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." Hester is somewhat shocked by the sight of her husband because he looks so different from the way he used to. His nature seems to have been completely corrupted by his attempts to torture Arthur Dimmesdale, the man with whom Hester sinned. He has become a devil after acting like one for so long.
About Dimmesdale, and Chillingworth's torture of him, Hester says,
"'Better he had died at once!' [...] 'Yea, woman, thou sayest truly!' cried old Roger Chillingworth, letting the lurid fire of his heart blaze out before her eyes." This "lurid fire" seems to refer to the fires of hell which have been kindled in his heart since he began to exact his evil vengeance on Dimmesdale.
Finally, Chillingworth says, "'I pity thee, for the good that has been wasted in thy nature.' 'And I thee,' answered Hester Prynne, 'for the hatred that has transformed a wise and just man to a fiend!'" The devil is often referred to as the "Arch Fiend," and so this quotation may be interpreted as Hester again comparing Chillingworth to the devil.