I honestly have no idea if it's:
A: Seriously a common trope in Korean Romance-Fantasy Light Novels that the minor antagonist rival "villainess" character has a tendency to die, often by execution by the male lead, and these isekai stories grew out of a desire to save these characters.
B: A trope specific to isekai stories that presumably started with whatever the early ones were that subverted the "reborn as the protagonist of a fantasy novel" by making it "reborn as the female antagonist of a fantasy novel" that was added to insert extra drama and stakes, since generally villainess characters in real Fantasy-Romance stories tend to have the advantage over the underdog female lead.
Whatever the case, it's a recurring trope in these novels and comics now. The main character is reborn as a villainess from a Fantasy novel she's familiar with, and tells the audience in her internal monologue something like "But I remember, the villainess Claudette Du Mal gets turned down by the male lead...and then excecuted at the summer solstice! Meaning I only have three months to avoid death!!" (end of chapter 1, don't forget to like and subscribe!). Probably at this point, it's copied from other isekai stories more than from Fantasy novels though.