This got darker pretty fast, but really some of the shit she has done to the mc is as bad if not worse than what we are seeing here happening to her. I wonder what the hell did Masaki do (if something) to make her torture him like that, right now he is the only person who treats her nicely.
Maybe Mikan was really messing with the "bully". We are seeing what Masaki does, don't forget that, she being a madwoman even back then would be a good twist.