Even though she came from an ancient family, she wouldn't be able to escape her punishment if she helped Luo Cheng kill Jiao Shuang.
Although the Jiao family wouldn't be able to do anything to her Han family, killing Jiao Shuang would go beyond her family's plans.
Her actions would lead to dissatisfaction amongst her family, and she would end up losing out greatly.
Her goal in being sent to the Dao Enlightening Sect was to try and suppress the Jiao family, but the Han family definitely did not want the matter blowing up too big. If Jiao Shuang died, the nature of this matter would have completely changed.
So Han Biluo was stuck in a difficult position. What should she do in this situation? Should she still war against Jiao Shuang, or should she just quietly retreat?
