Roger Chillingworth's character changes drastically throughout the course of Nathaniel Hawthorne's The Scarlet Letter. He is first introduced as the husband of Hester Prynne, a kind and gentle man. In part because of his physical deformities, Hester never experiences a heart-pounding, lustful love for him, but none the less she honors and respects his age and knowledge of the world. He seems to be a wise and stable husband, but after two years of separation from the woman he truly loves, he returns to find that his wife has betrayed their marriage and committed adultery. Hester promises to keep his identity as her husband a secret, and Chillingworth takes identity of Dimmesdale's personal physician. Slowly and methodically he begins to wear down the soul of Hester's lover. Chillingworth uses the knowledge of Dimmesdale's affair to drive the man who has fouled his wife into insanity. A man first seen as strong and faithful has become a sinister, plotting monster set out to destroy another human being.
Chillingworth is doomed from the moment he visits Hester in her cell and decides to devote the rest of his life to finding and torturing her lover. For seven years, Chillingworth keeps a close eye on Dimmesdale. He gives Dimmesdale medicine for his physical sicknesses, but mentally Chillingworth is chiseling away Dimmesdale's spirit. Chillingworth seeks his revenge in the most savage way possible- with prolonged, painful torture.
When Hester confronts Chillingworth and tells him that she can no longer keep the secret of his identity from Dimmesdale, she sees a different man than the one she married. His features have become even more deformed and she can see evil where she once saw goodness. She tries to save both Dimmesdale and Chillingworth from the punishment she sees Dimmesdale has wrought on both men by telling him that she was the one who wronged Chillingworth, not the opposite. However, she has come too late. Chillingworth knows what he is doing, but does not care. His fate is sealed.
At any moment, Chillingworth could have stopped his dreadful actions and atoned for his sin. Hester reveals her sin, but Dimmesdale is too sick to confess, instead he holds it inside. He knows that he is killing another man, not in cold blood, but in a worse way. He is destroying another man's psyche. Neither Hester nor Dimmesdale sinned out of hatred or evil, but Chillingworth knowingly destroys another without caring about his fellow man. This is a true act of both hatred and evil and because of his remorseless, planned actions, Chillingworth is the greatest sinner in The Scarlet Letter.