He had poisoned her. Why not increase the dosage to kill her outright, saving trouble and further highlighting Liu's viciousness? It would also make things quieter for him, removing a spy without any responsibility on his part.
But he didn't. Instead, he timely called a doctor to save her life.
Firstly, perhaps because he knew that if she died, Liu would certainly try to insert someone else into the situation, which he found troublesome.
Secondly, it also served as a warning, telling her that as long as she behaves, there will be peace.
He didn't want to deal with whoever Liu might insert next, hoping she could discern this and become a compliant person.
Su An'an sighed, never expecting the rumored sickly person to be so perceptive.
