Reverend Hale, introduced in act one (scene three) of Arthur Miller's The Crucible, is a dangerous man to bring to Salem. A minister from Beverly, Hale is known for his "experience in all demonic arts." Not only does he know of demonic arts, he found a witch in Beverly. Since previously having found one who admitted to witchcraft, Hale is considered an expert on witches.
Reverend Parris, in hopes of clearing his name, sent for Hale. Parris hopes that Hale is able to clear up any talk about Betty (Parris' daughter) being a witch. That said, Hale's presence only makes matters worse as other villagers begin to accuse others of witchcraft out of revenge. People will believe anything that Hale says because of his previous experience with witches. His presence simply makes witches more real.
Hale is also dangerous because he fails to stand strong in his convictions. He wavers in his defining of witches and worries about the truth's ability to survive among the truly guilty (like Abagail Williams).