According to Hawthorne's narrator, Roger Chillingworth had all his life been kind, though not warmly affectionate, and calm in temperament. He had spent his life as a scholar in solitary pursuit of knowledge, which is what led him to desire Hester's warming companionship to begin with, as he still cherished the hope of discovering the joys of love and affection, even though his scholarly pursuits still occupied him. The narrator clearly states that Chillingworth had always been "pure and upright" in his interactions with others.
However, as a direct result of the consequences of the Minister's and Hester's impropriety and indiscretion, Chillingworth underwent a great change of nature. He was "seized" by a "fierce" and terrible "fascination" that propelled him down a new path on which he was not free to follow anything but the fascination that gripped him. He fell from his pure and upright standing because of bitter disquietude over the wrongs inflicted on him that robbed him of what he believed was his one chance at the attainment of love and a warm affectionate heart.
After this fall, after having his worth chilled by the icy grip that seized him, Chillingworth became a "leech" of others' worth drained from their souls just like he would leech the goodness from the herbs in his medicinal studies. Those herbals were meant to be used for good, but the leeching of others' souls was meant for harm. Though his interactions with others were pure and upright at the start, even if preoccupied and cool, his interactions at the end were devious, manipulative, insinuating and harmful.