Pearl, insightful young girl that she is, warns her mother away from Chillingworth in Paragraph 23 of Chapter 10. She tells her mother Chillingworth is "the black man" which is another word for devil:
Hester Prynne, likewise, had involuntarily looked up, and all these four persons, old and young, regarded one another in silence, till the child laughed aloud, and shouted--"Come away, mother! Come away, or yonder old black man will catch you! He hath got hold of the minister already. Come away, mother or he will catch you! But he cannot catch little Pearl!"
In the sixth and seventh paragraphs of Chapter 14 - (page numbers vary greatly from edition to edition of the book) - Chillingsworth is described thus:
"...there came a glare of red light out of his eyes; as if the old man's soul were on fire, and kept on smoldering duskily within his breast, until, by some casual puff of passion, it was blown into a momentary flame...Chillingsworth 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".
Chillingsworth's evil obsession with revenge for the past seven years has changed him from the "intellectual and studious man, calm and quiet" whom Hester remembers, into something akin to a fiend. Chillingsworth also is aware of his own transformation, and considers it beyond his power to stop it.