I think the point of Bell possessing Olivia, besides the reveal of the man that is supposed to kill Olivia, is for it to lead to her confronting her fears in her subconscious mind. Fears that have driving her a lot this season and more so for her entire life. I notice a change in Olivia after her mind trip. Like she's more relaxed and at peace with herself than she ever was before. Look at how she told Peter about her future killer in a matter of factly kind of way. Nothing haunted or tormented. She seems oddly fine with it or confident that he'll be stopped.