I believe that the turning point in the novel One Flew Over the Cuckoo's Nest, by Ken Kesey, is when Billy and Candy are found the morning after their sexual rendezvous, naked, in the Seclusion Room. Nurse Ratched threatens to tell Billy's mother about his behavior. Unable to face such a situation, Billy takes his life while waiting in Dr. Spivey's office. Furious over this, along with his long-standing struggles with Nurse Ratched, McMurphy breaks into the Nurses' Station, and attempts to strangle Nurse Ratched. It is at this point that the direction of the plot shifts.
After his attempt to kill Nurse Ratched, she arranges for McMurphy's lobotomy. From this point on, the rest of the ward sees their leader and friend turned into a "mindless" shadow of who he was: McMurphy is gone. Seeing what has become of his friend, Bromden who cannot abide to see McMurphy brought so low, smothers McMurphy, picks up the tub to smash through the window, and makes his escape.
Although Nurse Ratched "wins," McMurphy's actions on behalf of his fellow-inmates is seen as a sacrifice, one of the themes of the book.