Ok! Based on Chapter 4, I believe that Sendai has sadistic tendencies, though she may be unaware of them herself. However, I think Miyagi suspects this too. it’s evident when Miyagi reflects, 'It's as if she's not herself,' referring to Sendai. Also, Miyagi seems to want to bring Sendai down to her level, metaphorically speaking. At this point, Miyagi is simply testing Sendai, but the orders she gives are becoming increasingly intense. If that makes sense but this is just coming from someone who hasn't read the light novel and this is just my rookie predictions. :)