why do you think tituba tells parris that the devil told her to kill him?

There are a few ideas at play here. When Tituba tells Parris that the Devil bade her kill him, she knows very well that Parris would take this as a veiled complement. Parris would take it as he is such an important man of God that the Devil would make a point of killing him. Also Tituba might be inferring that Parris is a mean and nasty as a master.