How do Danforth's and Parris' interests conflict with the common good of society and produce dire consequences for another member/group of society?

Both men need to validate themselves and their jobs. Parris is the "man of God" around town. The witch trials are a platform for him to showcase his piety and preserve his name. Parris freely indulges in Abigail’s witch calling around town. To seem Holy he lets others suffer. Indeed he is more than happy to denounce Proctor as someone who hangs out with the devil; Proctor had criticized Parris before. Danforth needs these witch trials and he needs convictions. Without them his court, his job and his identity means nothing. In the face of obvious hypocrisy and lies Danforth is happy to go ahead with the executions of clearly innocent people to keep his standing and employment.