In The Crucible, Reverend Parris fears that his congregation is out to get him. Because his appointment was disputed by some, namely the Putnams (they supported another candidate), and reviled by John Proctor (who won't attend because he feels Parris is a materialist who uses scare tactics in his sermons), Parris suffers from paranoia. Parris is insecure in his public reputation, and he should be: he expresses no discernible charity or love of others.
Mainly though, Parris is afraid that he will be implicated in the witchcraft accusations and scapegoating. After all, it is his daughter, Better, who is vexed. His slave from Barbados, Tituba, is implicated by his niece Abigail Williams. So, three of the four primary practitioners of supposed witchcraft in the woods all live under Parris' roof.
At first, Parris wants a medical explanation for his daughter's illness. But, after the doctor cannot find a cause, Parris goes along with Hale's suspicions and Abigail's scapegoating of Tituba. In fact, Parris is overly cruel to Tituba in order to deflect blame for bringing a tribal voodoo practitioner to Salem.
Reverend Parris was terrified by the events in Salem because the situation threatened his position as a minister in the community. Prior to finding the girls dancing in the woods and the chain of events that followed, Parris was already facing numerous challenges from the community. His appointment as minister was not readily welcomed by a section of the community because they supported a different candidate. On the other hand, a different section of the community resisted his authority because they believed he was selfish and high-handed in his activities as the minister.
Reverend Parris was suspicious of his congregation, and he believed they were out to oust him. He also thought that information about his daughter, niece, and servant dancing in the forest would be used maliciously against him. Thus, when an opportunity to accuse other members of practicing witchcraft presented itself, the reverend led the charge in order to deflect the attention from his family.