"become the evil you're afraid exists in the world, to justify hardening your heart."
It's very much the kind of motivation that's easy to arrive at in an armchair psychoanalysis fashion, and convince yourself it's true because it tells a complete story... but it's absolutely not the kind of thing a real person thinks to themselves ahead of time as their motivation to act. That's bananas.
When she isn't behaving like this, I think her characterization as a pragmatic not-that-mean Mean Girl is actually really well done
I've met people like that though, I don't think is bad writing. However, I thought part of the reason they behaved that way was because they were under the influence of media (TV shows and the like), you add to that some trauma, and you get a person with an overdramatic view on life.
If the point is to protect her psyche, she's clearly smart enough to rationalize her way out of the problem
I don't know how she would rationalize it differently. While smart, she's mentally unstable and a teenager who hasn't had any positive feedback on her worldview. FMC is a contradiction to her beliefs; being wrong wouldn't only hurt her pride, but it would mean that her suffering was meaningless in a way
She's in a loop, clenching tightly to denial. What you envision wouldn't be possible for now, because it would require her to have a different mindset, accepting that she just wasn't as great as she believed (she already thinks that, since she knows she's not her ideal self, but she believes that it is because nobody can be like that instead of being a problem with herself).