Not everyone who studies black magic wants to become the Dark Lord. Why is it that no one believes it when they say it? Did she think there weren't enough potions papers or that the history of magic was too easy to memorize? Why would she cause such trouble for herself?
Rosette Black was very distressed. She just liked dark magic. Apart from that, she had always been a good wizard who abided by the law. But since she came to Hogwarts to go to school, everyone seemed to regard her as a reserve for the Dark Lord.
Rosette doesn't want to be the Dark Lord! Being a Dark Lord means being disfigured! Not only will your face be disfigured, but your IQ will also be lost!
She just wants to be a quiet wizard who concentrates on academics, just petting the cat when she has nothing to do, and doesn't want to do that kind of thankless job.