Why does Santa Claus in Jing Yi pretend to be dumb?

Protect yourself. You is an action thriller directed by Jung Hoon Lee, written by Cheng Chuyi and starring Li Yingai, Jin Huizhuo, Guo, Li Hongnei, etc. The play tells the story of a 40-year-old female detective who used to be a policeman and an insurance investigator. Santa Claus pretends that his voice is some kind of evidence of relevance, so she must protect herself from being killed and cooperate with a female college student who is the murderer.