Nurse Ratched is a talented manipulator, often using pathos to convince the men on the ward to behave the way she wants them to behave. One example of this occurs in Part Four, when Nurse Ratched finds Billy and Candy after they have had sex:
“What worries me, Billy,” she said—I could hear the change in her voice “is how your poor mother is going to take this.”
She got the response she was after. Billy flinched and put his hand to his cheek like he’d been burned with acid.
Nurse Ratched has a close friendship with Billy's mother, and Billy has long struggled to gain independence from his mother. Sleeping with Candy is an important moment for him because it is the first time he has had sex. To him, it represents his independence and adulthood. He feels proud of himself until Nurse Ratched reminds him to be ashamed and to think of his mother's reaction. Bromden can tell that the nurse's words are calculated. "She got the response she was after," he writes. Billy is so strongly affected by the nurse's words that he goes into the doctor's office and slits his own throat, dying. She knew exactly what she was doing.
Nurse Ratched blames Billy's death on McMurphy, telling him that he drove Billy over the edge by arranging for him to have sex with Candy. It is clear to everyone that Billy's shame, spurred on by Nurse Ratched, is what killed him. By shaming Billy until he resorts to such a drastic act, Nurse Ratched turns him into a pawn in her war with McMurphy. Pathos thus functions for her as a tool of control. She is willing to use it even to devastating ends in order to get what she wants.