why is Reverend Paris fearful?

The Reverend Paris is a greedy little guy. He worms his way into getting what he wants. Arthur Miller did his homework on Rev. Paris and concluded that he didn't have to make him any nicer in fiction. To answer your question Rev. Parris was afraid of losing everything that he worked so hard to worm out of the community. All his golden candlesticks and even the deed to his house. Rev. Parris feared losing his standing in the village. He had a pretty cushy job. With all the witch stuff happening on his watch, things didn't look good for him. Accusations get flung around and certain people become a threat to him. In particular, John Proctor calls him on his sliminess. Parris fears John and, of course, accuses him of hanging out with the devil to get rid of him.