Roger Chillingworth's most horrible sin is this: In seeking revenge, he sets out to deliberately and methodically destroy another human being, Arthur Dimmesdale. When Chillingworth comes into the settlement to find his wife standing on the scaffold with another man's baby in her arms, his shock turns quickly into a lust for retribution. When he meets with Hester in the jail after she is taken down from the scaffold, he extracts from her the promise that she will not reveal his identity. His intent is clear; protected by the veil of anonymity, he intends to discover and pursue Hester's partner in sin. Hester's husband's plan for revenge has already formed in his mind, and he takes up his new false name, Chillingworth.
Living in the village as the physician he is, Chillingworth waits and watches, obsessed with determining which man fathered Pearl. Once he suspects the minister, he moves in with Dimmesdale, pretending to be his caring physician. Chillingworth uses his position to probe Arthur's conscious and subconscious mind, tormenting him psychologically, and perhaps, it is implied, poisoning him physically. When Chillingworth discovers the "proof" he has sought (something shocking appearing on Arthur's chest--a second scarlet letter?), he dissolves into a state of complete moral corruption, overcome with joy at the pain and suffering of another. Chillingworth has not only betrayed his morality as a human being, he has corrupted his profession, using his skills as a physician to destroy rather than heal.
Until the novel's dramatic conclusion, Chillingworth uses all his power to keep Arthur from making a public confession. So long as Arthur hides his secret sin, he remains trapped in Chillingworth's devious daily torture. When Arthur finally does confess, standing on the scaffold with both Hester and Pearl, he frees himself from the physician's grip, as Chillingworth acknowledges:
"Hadst thou sought the whole earth over,” said he, looking darkly at the clergyman, “there was no one place so secret,—no high place nor lowly place, where thou couldst have escaped me,—save on this very scaffold!"
The great irony of Roger Chillingworth's revenge, however, is that Dimmesdale grows in spiritual insight and achieves peace, whereas Chillingworth wastes his own life, destroys his own integrity, corrupts his own soul, and spends what's left of his life trying to make up for his most terrible sin.