I hope it is out of the realm of what most of us could do, but I wonder what happened before, between her and CS. I know a person who hated me so much she committed a very stupid thing, essentially, a federal crime. (She never saw me in her life, and had I wanted to, I could have totally ruined hers for this). How can a person who never saw you hate you? She blamed me on giving an advice to her bf, which i did not provide at all, as i didn't know he had a gf. But - all anger, pain, whatever he caused her, and probably, was significant, she turned on me. We, humans, don't want to see the reality, and instead of saying "he doesn't love me", she fell for his explanation. "If it weren't for her, we'd get married and be happy".
I think any of us might remember the situation of some women hating you for nothing - merely because they can not accept the harsh reality, "he doesn't love me. He won't marry me". Instead, it is "his parents don't like me, he doesn't want to be in therapy, he doesn't trust women because of his ex, this woman is stalking him and threatening suicide", etc, etc. And guys try to avoid drama, so they lie. In KA's situation, it was extreme, and one wonders how did she imagine her future life with CS if she managed to pull it through? Per her father, CS was not the epitome of loyalty. What would she do with the next pretty girl in his scope? Kill her, too? But, perhaps, there were some traumatic details during the time KA and CS broke up in the autumn of 2021, that we don't know about and that altered her thinking.