How and why does Chillingworth become more and more vicious and diabolical, especially towards Dimmesdale?
Chillingworth, Hester's actual husband working as a doctor under an assumed identity, became Dimmensdale's physician and moved in his house. In time he began to suspect that his patient's maladies were connected to Hester. Slowly, he starts a psychological game with Dimmensdale, for he realized that Dimmensdale is emotionally weak. He would prop him questions and comments related to confession, and about telling about one's sins as a way to get him to confess or give more information about the situation. When he saw the letter A carved in Dimmensdale's chest, he realized that this was the man he had wanted to venge for years. Although his hatred for this man is atrocious, he takes his time and starts his revenge by basically eating up Dimmensdale from the inside out. He knows about Dimmensdale's self mutilation by whipping, and he adds up to the poison brilliantly. Eventually his anger gets worst as he finds out Hester's plans to leave for Europe with Dimmensdale and Pearl. Chillingworth goes all the way to booking himself in the same boat for Europe, but his plans to continue torturing Dimmensdale failed when Dimmensdale confessed to his adultery with Hester in front of everyone and died on the spot.