Xu Jing's mother had sustained internal injuries after being beaten up by the local government officer. This so-called internal injury was very much dire. If proper treatment is given in a timely manner, Xu Jing's mother would not need to face the music. However, her mother did not want to burden her again due to this injury, and decided to hide the truth. Eventually, the internal injury deteriorated and took her mother's last breath.
Indeed, Su Li did not know anything at all, not realizing the extent of the internal injury. The last thing that crossed her mind was death. It was obvious that she wanted Xu Jing's mother to die at all costs. Moreover, this move was to set Xu Jing free in order to get involved in a couple relationship.