I am a little late in the game but I finally watched this to complete my viewing of the entire franchise. I have to say I was going into this movie with little expectation given that I had already heard such mixed, mostly negative reviews and that was why I procrastinated watching this. I kept an open mind none-the-less.
Some things confused me because I have known Michael to be super human if human at all. So seeing him weak... I couldn't figure out how or why. I also always felt that Laurie had to die for Michael to either die or become weak enough to kill sort of like a Voldemort-Harry-Horcrux scenario. So those were and are my major issues at this point. Also, not seeing his damn face but whatever.
I think the whole point of Corey was that even though the killing of that bratty kid was an accident and he was a good person he did have a dark side. As anyone does, there is always that borderline of good versus evil in everyone but how you control it is who you are. Any good person can turn "bad" under circumstances. There were moments where you can see Corey's anger coming out and how on the edge he was. I think that called out to Michael and that's what he fed off of. In that moment when they met they connected and as Laurie was narrating- evil consumes you. That brought out Corey's evil so in a way yes, Michael possessed him and Corey wanted to learn and kill. Be the monster that the town has made him out to be. This "possession" happened in touch.
When Laurie and Michael had their showdown in the kitchen at the end, Laurie is holding Michael's hand as he seemingly dies. Again, Laurie narrates while typing her book and talks a lot about evil, being inside people and consuming them etcetera.
At the very end she has the mask on her coffee table. I think this indicates that yes, Michael is dead but the evil that consumed him and then Corey (whole point of bringing in his character to show the possibility of this "possession") lives now in Laurie. So it isn't a happy ending for her character because will this evil consume her too or will she fight it.
This is how I understood the movie and honestly, if this is at all the point they were trying to make, then I would say this was a decent movie and "end" to the franchise.