I could've sworn, in the movie and the book, the devil was after Merrin. It had fought with Merrin before and called Merrin's name out when Merrin appeared at the McNeil home. I think it was looking for showdown #2. I don't know the working dynamics in hell, but it seems the demons have to possess a regular human host as a means of confronting the priests. If you mean, at the end when the demon jumped out of Regan's body into Father Karras', that was a means of convenience, I think. The girl's body was wasted, probably on the verge of death, and getting pummeled by Karras. I guess you can say Karras literally beat the devil out of her. lol If you watch Exorcist 2, you'll find it is actually the death of Merrin the devil relishes the most. Anyway, that's my opinion.