Mis-blamed - While she has her flaws as a character, a lot of Shirley's scrappy status comes from people thinking that she willingly set out to destroy the world and Senel due to him rejecting her. In truth, she had lost the memory of what becoming the Merines would mean, and she becomes it to bring people happiness; only during the transformation, where it's too late to do anything about it, that she remembers that Nerifes wants humanity wiped out.
Shirley: I see...so that was it. I remember now, Fenimore...why I failed the Rite before. I realized what would happen if I completed the Rite of Accession. So I refused. I failed on purpose. If I completed the Rite...If I awakened as the Merines, it would mean......That I would kill my brother.