Starlight used to be a horrible pony. Enslaving an entire town as its dictator? Preventing the Sonic Rainboom from ever happening? Nearly destroying Equestria as we know it? Very few things come close. But there's one thing Starlight is certain of: she's still the same magically-gifted, intelligent pony, and she hasn't lost sight of herself in that regard.

So then why does Twilight believe that Starlight was acting irrationally up until the point of the latter's redemption? You may agree with that assessment, but not Starlight, she even takes offence to this kind of allegation. She wasn't somehow "less rational" when she was a villain. Right?