Not going to give away the whole plot, but I found making Corey into a secondary villain really took away from the dynamic they'd built over the last two movies between Laurie and Michael, to the point where Michael was basically a side character in the structure of this movie.
They played it up to a place where his murder spree from the last two films is sort of looming over Haddonfield, trying to make it feel like he is omnipresent in the film even when he's not there, and it just didn't work for me. Once he finally shows up though, it's sort of back to business as usual.
My biggest beef is that I just don't care about Corey. He's set up as a lousy babysitter who can't follow the parent's instructions, which leads to a situation in which an accident occurs, which then leads to him being a sort of social pariah in town. He's awkward as hell, and about as charismatic as a tree stump, so I kind of just don't care about the guy. I don't feel bad watching him turn into a murderer, and as a secondary villain in this series I'd just as soon he got the hell out of the way so I could have more of the villain I wanted to see.
In the end Michael is very definitively gotten rid of, short of a full supernatural turn that the series isn't likely to take. But, it does make nods to the idea that Michael has an evil power meter, for lack of a better term, and that something of that is transferred to Corey.
I dunno. It's not a terrible movie, but it felt too long and didn't really payoff in the ways that I'd hoped it would.