Why did Sister Mei in the darkroom go bad?

Sister Mei in the darkroom went bad because of greed in the movie. In the play, Sister Mei is an insatiable shop assistant with easy work and high salary. So most of the things she did were against Liu Wenjing's wishes. On the surface, she is still thinking about Liu Wenjing, but in fact, she has her own purpose. The suspenseful criminal investigation drama "Dark Room" produced by the Golden Shield Film and Television Center of the Ministry of Public Security is adapted from a real event, which tells the story of judicial policemen Jiang Chuan and Zhang Yang fighting wits with criminals in the process of investigating the campus loan incident.