Ayumi has a textbook superiority complex "I'm better than anyone at Chidori, surely" built to mask her feelings of inadequacy from living in her accomplished family's shadow. But she herself can tell it was on shaky grounds to begin with, and that's interesting.
I said this before with Subaru, but someone who knows if and when they did something wrong and feels guilty over it can't be a truly bad person. IMO, truly bad people never even realize things like this; they also often feel they're right and may even enjoy it. Ayu clearly agonized over what she did.
Rintaro apologizing for Ayu finding herself far out of her comfort zone... wasn't quite the final blow I thought, but maybe Kaoruko here will deliver the coup de grace of kindness, so Ayu can finally start down the path of friendship; (I'm confident Madoka is joining given how excited she was over this, but I want to see at least one more Kikyo girl in their friend group)