Bc that was her last wish as she died, she knew that she was not smart enough to avenge herself. Also tbh the princess in my opinion was never badass as we aren’t shown what truly happened in all the scenes of her that we were shown.
I mean. She is not actually badass, but act as one. Also, how can she be not smart? She was the eldest princess and she knew a lot things based on the words of her father and other officials. They said she needs to "retake" classes. And even if she wasnot sure abt her brain and logic, she still could have other options to at least stay still with the demon in form of a soul and so on, but she did not. She just "dissapeared".
Ok. This story already began messed up.
Firstly it was interesting seeing this princess being badass, but then it became queickly soooo boring. She just without any plan, or idea how would the demon achieve threw all her wishes into the face of the demon