But if it wanted to kill me, why did it possess me? Unless it wanted to finish you off first.
I can tell you that thing was very powerful and Grace would not have been able to eject it from her body on her own. Sometimes, if an individual has a strong enough mind, they can force out whatever is possessing them, but it also depends on how powerful the possessor is. She might have been able to regain control temporarily, but it was too powerful to simply eject. Once she regained control I was able to use my own powers of possession and dive through her and physically push the thing out.
But as soon as I become solid it was able to enter my body. Using my own ghost powers I was able to reach inside my body and pull it out. I guess the thing didn't count on me being able to do that