How are characters such as Hale or Parris responsible for the trials?
I think that part of the beauty of Miller's construction of the evil in Salem is that not one singular person owns all of the responsibility for what happened. Each individual owns their own share. This is the case with Hale and Parris. In the case of the latter, his own insecurity and fear about his own position as leader of the clergy enabled him to embrace the "easy" solution of witchcraft. He advocated this as long as it benefited him. In the final act of the play, Parris recognizes that his credibility has run out with the town, and that he becomes the target of threats and intimidation. In the end, Parris ends up running away from his own responsibility as he leaves the town "on the highway" with no one hearing from him again. Hale's responsibility is a bit more complex. An outsider, Hale was brought in to further the case and cause of witchcraft, a job which he dutifully performed. He does realize that he was used as a pawn of those in the position of power, a revelation that makes him want to abandon his previous support of what was happening in Salem. Unlike Parris, Hale never forgets his own role or his own guilt in what was happening in Salem and how he bears some level of responsibility for it. In this, one sees his own motivation for wanting those accused to lie and offer a confession to save their own lives, and, in the process, alleviate his own guilt for what he had done.