Billy commits suicide by slashing his throat with some instruments he finds in a doctor's desk. He kills himself because Nurse Ratched humiliates him and threatens him with his greatest fear - his mother's derision and disapproval - after she catches him with a prostitute.
In an elaborate scheme, McMurtry had arranged to bring Candy, a call girl, onto the ward so that thirty-one-year-old Billy can have his first experience with a woman. Billy has a sick, almost incestuous relationship with his mother, who is ironically a good friend of Nurse Ratched. Billy's mother exercises complete psychological control over him, regularly leading him by the hand and caressing his head in her lap. When he tells her his dreams of going to college and finding a wife, she scoffs, saying he "has scads of time for things like that", and when he points out his age she protests in disbelief, telling him "do I look like the mother of a middle-aged man?"
The author is clear in demonstrating that it is his mother and the institution she supports that are causing Billy's mental illness. Billy gains a modicum of confidence after his experience with Candy, but Nurse Ratched quickly and ruthlessly knocks him down by striking at his weakest point. Unable to bear the thought of being shamed before his mother, Billy kills himself (Part 4).
Billy Bibbit kills himself by slitting his throat with doctor's instruments after Nurse Ratched makes him feel guilty for sleeping with Candy. Billy is connected to his mother to the point of having an Oedipal Complex, and his mother, who is friends with the nurse, controls Billy to the extent that he has never had a girlfriend. McMurphy arranges for Billy, who is thirty-one, to lose his virginity to Candy, but when the staff on the ward find Billy and Candy together, the nurse makes Billy feel incredibly guilty. She tells him that she will inform his mother of what happened, and Billy blurts out that McMurphy and the other men are to blame for his sleeping with Candy. As a result, Billy feels guilty and ashamed of what he has done and about getting his fellow ward-mates in trouble. As a result, he slits his throat while in the doctor's office.