I had to reply it as well, I initially thought he had an extra sword hidden but if you watch them both laying…
After they both fell down and you can see that she was stabbed at her lower right arm. There was no way she could have stabbed herself that way. Also, that wound could not have killed her. It was not a deadly wound. The answer is he stabbed her, but the director didn't show us that scene.
https://mydramalist.com/687393-the-prisoner-of-beauty