Why does Rebecca nurse leave the room when reverend hale is there

She doesn't want to watch Betty be exorcised.

Rebecca is the wise and calming presence that is needed in the midst of all the mayhem that is taking place in the play. She argues that all this talk of witchcraft is nonsense, and that the girls are simply being silly and tricking the adults. However, no one listens to her.

Therefore, when Reverend Hale is brought into the room, Rebecca leaves claiming she wants nothing to do with it. She does not want to be involved in this business, because the Reverend makes her uncomfortable, and she believes that playing with spirits is dangerous and unnecessary.