[If you mind spoilers, please scroll away.]

My theory is that Monika can control what the girls do and say.
You know that note in the poetry book Sayori left, saying “GET OUT OF MY HEAD”?
I think Monika was hacking into her file and saying things to her. That was when Monika was making Sayori (And I quote) “More and more depressed”. Something She admitted to in the fourth act of the game “Just Monika” as I like to call it.
Also, you know the white text with a bold black outline that Yuri and Natsuki said crazy things in? I think that was Monika getting into their heads. Monika was saying that. Mostly Yuri, because she also admits to this in “Just Monika” Saying she was: "Amplifying Yuri's obsessive personality”. And I also think when Yuri confesses to the Protagonist, Monika gets mad and makes Yuri Stab herself to death. So clearly She can Control the girls. Also, I know they all like the protagonist, but The literature club is what makes Monika (And Later, Sayori when Monika Restores The girls’ files.) Obsessed and willing to do anything to be alone with the protagonist, even if it means killing everyone who gets in their way. Being the president of the literature club, simply put, corrupts anyone with even a minor crush on the protagonist.