I think it got cleared up with the last two updates. She explains it pretty transparently, how she got hurt in the past, how much the MC resembles her ex, and how much she hates his approach because of that. So I'd argue that her bitchiness is very clear now, however after having their conversation in the Mess Hall, her behaviour towards MC definitely changed. I agree a gag in his mouth and chains around his arms and legs might have been a bit too much, especially if we start wondering why she has them in the first place, and who she was planning to use them on, since she already had them in her personal safe, before MC came into play. Still, after an invitation to her room for a hot sex with badass military girl, with a creampie finish and a gentle kiss to top it off, you gotta admit that her bitchiness diminished significantly.