Ku Waowen was pleased at seeing Lin Mu fall into her trap. While it had taken quite a bit out of her to plan it out and execute it, she had finally succeeded. The mental pressure of Lin Mu having information about spies was simply too heavy.
She knew that there was no way she could kill him, just like that. Especially since her earlier attempt at sending a message to her superiors had failed due to Lin Mu interrupting. While she had indeed added the content that she needed to send, she did not know if it was sent out successfully or how much of it was sent.
Either way, she knew that only by keeping Lin Mu alive and extracting information from him was the option for her here. Any other option would lead to grave danger on their end.
They would not know how their foes were planning against them. In a prolonged war, the most important part was information and once someone had a grasp on it, the war was as good as half won.