Explain the irony in using the word "leech" in association with Chillingworth in Hawthorne's The Scarlet Letter.

Roger Chillingworth, the cuckolded husband of Hester Prynne in Nathaniel Hawthorne's The Scarlet Letter is the greatest sinner of all the characters.  For, he enters the home of Dimmesdale under the pretense of being a healer of the body when, in truth, he desires to "violate the sanctity of the human heart" by probing into the conscience of Arthur Dimmesdale.  While pretending to be the friend and confidant of Dimmesdale, he seeks to discover if the minister is the man who has sinned with his wife.  Like a leech, he attaches himself to Dimmesdale, and through his insidious and surreptitious questions he seeks the knowledge that is in the heart of the unsuspecting minister. His drawing forth from the ailing psyche of Dimmesdale is anything but healing; instead the minister is weakened the longer that the physician dwells with him, growing paler by the day.  In Chapter XIV when Chillingworth and Hester talk, he tells her with a fierce pride of his torture of the minister.  Describing what has taken place he says,

A mortal man, with once a human heart, has become a fiend for his especial torment.

Chillingworth suggests further that, rather than having paid his debt, Dimmesdale has increased it, by making of Chillingworth the fiend which he has become.  He is guilty of what Hawthorne calls "the unpardonable sin":  the subordination of the heart to the intellect.  It occurs when one is willing to sacrifice his fellow man in order to gratify his own selfish interest.  This interest is to destroy Dimmesdale; the promise he made to Hester in their interview of Chapter IV:  "He will be mine!"

 And, here lies the irony:  leeches were placed on ailing people in order to draw out the bad blood which made them sick.  Thus, leech became synonymous with physicians of the 17th century since this method was employed often by them.  However, this physician, the leech, Chillingworth, drains the very soul of Arthur Dimmesdale in order to learn its secrets, further debilitating him. Like the leech, Chillingworth's soul is the blackest, for his revenge is dependent upon the destruction of another human being, not the healing.