why does Tituba say "he say Mr. Parris must be kill! Mr. Parris no goodly man...and he bid me rise out of my bed and cut your throat!"

Tituba is no fool. She knows there isn't away out for her. She is a slave girl and Abigail's haphazard condemnation is enough to make her a scapegoat for this whole thing. Tituba instead confesses readily and names other people, like Mr. Parris, "allegedly" involved. This takes the attention off her and puts her in better standing with the court.