Funnily enough, this is what a fair few duchesses and the like point out during the usual engagement annulment in short stories on Syosetu, even without them being reincarnated assassins.
"I'm a duchess, potato girl. Why would I bother ripping up textbooks when I can frame you, or have you killed, or kill you socially so thoroughly that even your fellow commoners won't talk to you for the rest of your days?"
But for longer stories, even though it makes little (not no, but little) sense, her still being alive, entitled, and annoying is what drives the plot forward.