Chillingworth, or Roger Prynne, enacts psychological warfare on Arthur Dimmesdale because he feels that Dimmesdale has cuckolded him. Chillingworth is jealous of the love that Hester has for Dimmesdale and disgusted that Dimmesdale will not confess that he is Pearl's father.
His acts of psychological torture against Dimmesdale includes becoming his medical adviser, seen in chapters nine and ten. The two men move in together and spend a great deal of time in each other's company. Chillingworth works to win Dimmesdale's trust, hoping that he will unburden himself and confess his sin. They have long conversations about the sins of other men, and Chillingworth opines that men's spiritual sickness manifests physically, implying that Dimmesdale's fading health is the result of his hidden sinfulness.
Chillingworth never goes so far as to overtly accuse Dimmesdale, but he becomes adept at playing on Dimmesdale's psychological weaknesses and persistently trying to get him to admit what he has done. Since Dimmesdale is a Puritan and deeply understands their beliefs about how God punishes sinners, Chillingworth's subtle manipulations are effective. His cruelty intensifies the anxiety Dimmesdale feels about eternal punishment, to say nothing of the guilt he feels about leaving Hester and Pearl to fend for themselves in a hostile community and his hypocrisy in leading a congregation.
Roger Chillingworth tortures Arthur Dimmesdale by continually questioning the cause of his mysterious affliction and neglecting to clean Dimmesdale's self-inflicted wound on his chest.
Initially, Chillingworth suspects that Reverend Dimmesdale is Pearl's father and offers to help cure Dimmesdale of his mysterious illness. Roger Chillingworth even offers to become Dimmesdale's roommate, which allows him to investigate Dimmesdale's dark secret. He eventually discovers the self-inflicted letter "A" that Reverend Dimmesdale has carved in his chest and proceeds to psychologically torture him. Chillingworth is obsessed with vengeance and conspires to torture the sick reverend by gradually provoking his guilt.
As the novel progresses, Chillingworth develops into an absolute fiend who refuses to let Dimmesdale out of his presence. After Hester informs Dimmesdale of Chillingworth's true identity and evil intentions, he agrees to leave the village with Hester and Pearl. In addition to provoking Dimmesdale to contemplate leaving Boston with Hester, Chillingworth exacerbates Dimmesdale's illness by mentally torturing him, which eventually results in his death.