A close association develops between Chillingworth and Dimsdale. Why does Chillingworth seek this association?

The use of the term "leech" to describe Chillingworth is at once appropriate and ironic. After all, he is a physician, and leeches at the time were used in order to facilitate bloodletting. At the same time, however, Hawthorne is obviously suggesting the parasitic relationship between Chillingworth and Dimmesdale. We return to our earlier postulation that Chillingworth goes after Dimmesdale not because he is a stock character or out of any sense of moral purpose, but rather in an effort to absorb the reverend's virility, to steal his life force and appropriate it as his own, both in vengeance and for his own sake. Chillingworth realizes that he is old, deformed, and unworthy of Hester, even though he is her husband. Yet, he seems to retain the unconscious desire that if he can somehow capture Dimmesdale's spirit, he will be able to gain Hester's love and allegiance.

It is odd that some of the townspeople can sense that Chillingworth may be on the side of the devil. As a matter of morals, we would expect them to side with the cuckolded husband, if they knew his true identity. But for all their strict laws and overreaction to sin, these Puritans can sense the energy of injustice that is growing in Chillingworth’s psyche; they are attuned to it. Thus society is split in half over the man, some seeing him as a helper of Dimmesdale, others seeing him rightfully as the spawn of "The Black Man," having dangerous motives.

He is evil and also suspects Dimmesdale of something. Chillingworth, being a "doctor", sees that he can have much power over the priest who is weak in mind and body.