@meijune
It's kind of a trope already, but mostly through "knowledge". I mean, it's an old truth, knowledge is power. And in this type of otome isekai, the transported person knows EVERYTHING about the romantic target. It's an unfair advantage.
Reminds me of another one I read. The fiancee/villainess dedicated her whole life to make herself worthy. She knew his favorite food...or rather, she knew his "favorite food at the castle", the thing the Prince was supposed to eat. Meanwhile, the "heroine" knew he liked meat skewers in the commoner market, though he'd never admitted it cause it wasn't befitting.
Having someone suddenly know you that well can be powerful. As I said, unfair advantage.
That's as far as to "how he turned into a Lilia obsessed idiot". As for why he's an asshole in general, tho...because he's an asshole. Period.