Intro. she cares about is her own personal gratification. Nothing else matters to her, especially not the thoughts and feelings of other people. She doesn't use any form of rationalization to convince herself that she is justified in harming others; the pure enjoyment she gets from doing so is reason enough.
With that said, Hoshiko understands the importance of keeping her urges and actions secret from others, so she has mastered the art of being a social chameleon and pretending to be a normal, well-adjusted person. It is only when she is alone with a victim that her true, sadistic nature is revealed. The other bullies are the only people that she feels comfortable being "her true self" around.
When Hoshiko and the rest of the bullies are ganging up on a victim, Hoshiko often tries to convince the other bullies to do things like burn their victim with lit cigarettes. The other bullies always have to talk her out of it. If Hoshiko was the boss instead of Musume, each of their victims would