I think the line is killing human, which is what the start of chapter 6... Also Senpai is kind of sus, or angel is really oddity since she can see fallen angel without monocle
Going by the Bible, Lucifer and a third of the angels fell when they started a rebellion against God, not when Lucifer was thinking bad thoughts. Mayhap it is the same here in this story.
Japanese-manga-Christianity is weird stuff. But most or all cultures that import Christianity arrive at strange concoctions, and Japanese fiction plays as fast-and-loose with Christianity as our fiction does with the religions that we classify as “mythology”.
A lot of secondary or apocryphal literature describes the rebellion in some detail, but the works ordinarily regarded as canon are rather vague. Perhaps thoughts themselves were regarded as rebellion. Certainly the Bible treats some thoughts and desires as violations of G_d's law.