I wonder if that thing saw me as a threat or nuisance?
Perhaps tha thing wanted to possess you in the first place and used Grace to do so? I think it was easy for the guy who stabbed her to, well, stab her and this being used her as a vessel. Then it used her to command the werewolves to get you and it waited for the right moment to possess you. But it didn't count on you being able to fight it off.