To seal the evil she must have the power to do it but when the seal weakens the evil within changes/transforms her body as a sort of price from what i got reading it
Tbh the moment Samurai was literally have losing flags all over him but I really rooted for the MC was when Samurai was planning to give Mary away to his grandfather 💀
Hmm maybe she thought about it at first that she really almost killed a person and that if she were to fight back its only gonna get worse for her. IDK what the author was trying to do putting in that character