She is almost definitely not one of the maid staff, or a professional. neither would have called the MC by their first name before trying to kill them (the staff would likely use honorifics and titles out of habit, and Scarlet would have recognized her, a professional would just kill her in her sleep and leave without climbing on top).
I feels like she was waiting for the MC to wake up to see her reaction, so almost certain it was personal, and the idiot who though she was going to be the next queen (if the king approved the replacement, which I kind of doubt he would, but we have not seen the king yet) then ran away from the fight seems likely. It could be someone from the other families implicated for crimes, but this kind of feels to personal.