两个人吃一个奶一个不吃了

The reason why Ning Cheng didn't take action immediately was because he knew that Xian Mao didn't want to kill Zhui Niu at first. If Xian Mao wanted to kill Zhui Niu from the beginning, Zhui Niu wouldn't even be able to call for help. Later, after chasing the cow and escaping, Xian Mao took action when he called for help....