Rosemary will never learn until her stepmother stops enabling her and her growing debt of consequences blow up in her face, all at once. The villainess reputation was only a matter of time considering Serena doesn't actually care to put up appearances that curry favor among people. I'm more surprised she wasn't considered a villainess until now.
Serena should have continued to leave home and be a full-time assassin. Staying there was a mistake when her only other ally the duke is never there and might as well not exist* (*the duke's money is effectively his representative, that everyone including Serena benefits from).