Chillingworth sets out to torture Dimmesdale and get his own revenge. His actions are premeditative and precise, looking only for his own personal vengeance. Hawthorne labels him as the worst sinner because of the selfish, harmful motives behind his actions. Even though Dimmesdale and Hester committed sin in the eyes of society, theirs was a sin of passion and love. This is why Hawthorne labels Chillingworth as the worst sinner.
Chillingworth becomes the greatest sinner as a result of his intention to do the greatest possible harm to another human being that he can. He doesn't want to expose Hester's fellow sinner so that the sinner's actions subject him to the law. He doesn't want to kill him either: to kill this man would be quick and relatively painless compared to the intense pain Chillingworth can create by finding out the man's secret and then torturing him with it. In his initial interview with Hester, he says, again and again, that the father of Hester's baby, the man who wronged both Hester and himself, will be "'mine!'" He plans to find the man and torture him with his sin.
When Hester and Dimmesdale sinned, on the other hand, they were not trying to hurt anyone. However, Chillingworth's entire existence begins to revolve around his obsession with finding Hester's co-sinner and ruining his life. He even presents himself as a doctor in order to be able to get close to his suspect, and once he begins to think that Dimmesdale is his man, he uses his intelligence and position to get as near to him as any man could. In such close proximity to his victim, Chillingworth can take every opportunity to bring up Hester and the child, the nature of guilt, and so forth, in order to keep Dimmesdale sin constantly in the minister's own mind. His intention to possess and ruin another human being makes him the worst sinner.