Gu Ning already achieved her aim, so there was no need for her to stay.
The amount of drugs wouldn't kill Qi Ziyue right away, but he would be tortured for a long time. What was worse was that it was possible that he would hurt himself because of the hallucinations caused by the drug.
Gu Ning didn't care about him at all, and only felt happier when Qi Ziyue was in a lot of pain. If Qi Ziyue died, the revenge for Tang Aining's death would be successful.
Gu Ning wasn't worried that Qi Ziyue might call the police and report her for keeping drugs, because nobody would believe the account of a man who was on drugs. In addition, the Tang family was searching for Qi Ziyue right now, so he didn't dare to show up in public.
Qi Ziyue had lost his reason now, and Gu Ning was afraid that he might hurt Chen Yunlin, so she went back to the study and untied Chen Yunlin, then she locked the door of her study from the inside. Afterwards, she left from the window.