Ouyang Biya was the one who snuck behind Ximen Qiwen. She told Tang Shaoyang that she would be ready outside, while in fact, she had been observing what happened inside the reception hall from the start.
The moment the chance arrived, she immediately took action. With her current power and the training she had gone through with Mathias, she easily killed Ximen Qiwen, one of the masterminds behind her family's fall.
If there was something she regretted, it was because of how easily Ximen Qiwen died. She felt like a bit of torture would do the work, would give her more satisfaction, but she didn't want to miss the chance.
She made a move without asking her Master, but the timing was perfect because her Master was fighting them. If she waited, she might miss the chance, and there was a chance her Master would stop her even though it was part of their contract. That was why she made the move.
