Why is Roger Chillingworth referred to as the leech?

Roger Chillingworth is referred to as the leech in "The Scarlet Letter" because he attaches himself to Arthur Dimmesdale and feeds off his emotional and psychological suffering, much like a leech feeds off blood. Chillingworth's relentless pursuit of revenge and his torment of Dimmesdale drain the minister's vitality and contribute to his decline. Thus, the nickname emphasizes Chillingworth's parasitic and manipulative nature.