Why did Ponyboy try to assume responsibility for stabbing and killing Bob?

In Chapter 11, Randy Adderson stops over at Ponyboy's house to see how he is doing and discuss what they will have to talk about in court the next day. Randy tells Ponyboy that he doesn't have anything to worry about because it was his friend Johnny who had the knife. Pony says that he had the knife and killed Bob. When Randy tries to correct him, Ponyboy continues to claim responsibility for Bob's murder....

In Chapter 11, Randy Adderson stops over at Ponyboy's house to see how he is doing and discuss what they will have to talk about in court the next day. Randy tells Ponyboy that he doesn't have anything to worry about because it was his friend Johnny who had the knife. Pony says that he had the knife and killed Bob. When Randy tries to correct him, Ponyboy continues to claim responsibility for Bob's murder. Ponyboy begins to shake and keeps repeating "Johnny is not dead" (Hinton 165). While Randy is leaving, Darry tells him not to talk about Johnny in front of Pony again, and mentions that the doctor said Ponyboy was "racked up mentally and emotionally." (Holden 166) Ponyboy has suffered a traumatic experience and has convinced himself that Johnny is still alive. Ponyboy's defense mechanism is to pretend that Johnny is still alive to cope with the loss of his good friend. He is not thinking clearly and assumes responsibility because of his confused mental state. The next day, the boys go to court and Pony is not questioned about the events that transpired the night Johnny stabbed Bob because his testimony would be invalid, due to his mental state. Ponyboy would be diagnosed with Post-Traumatic Stress Disorder if he were to be psychologically evaluated.