Since the beginning of this arc the author focuses on Mizuha and, in my opinion, gets us to feel sorry for her. Even though many readers would still hate her since she is "perfect".
But the monologue with her thoughts combined with her actions just throws all that sympathy out the window.
Fans have a strong feeling that the knocker is still around and it very well could be the reasons for the actions that happen this chapter.
But as far as I can recall knockers don't really control the thoughts of their hosts, they can communicate but don't take over mentally. Granted everyone in this family has psycho problems but two of them, the little girl and the male heir (can't recall their names) didn't seem as crazy.
Who knows, since readers suspect Mizhara to have a knocker the author is giving us a red herring with this and the "I wanna die" bits. Maybe the knockers are gone and Mizuha is just nuts. 
This has turned into a rant but just something in these last few chapters have really rubbed me the wrong way.