Why do you think Ponyboy insists that he is the one who killed Bob? How might this help Ponyboy feel better?

Like many people who have lived through a traumatic experience, it is possible that Ponyboy is suffering from denial. He refuses to beleive that Johnny has died. He even becomes angry with Randy when he visits to give his condolences.

We could probably infer that Ponyboy's refusing to beleive Johnny was dead was his not wanting to cope with the death of yet another loved one. Even after Johnny's death, Ponyboy is still compelled to protect Johnny, just as he did when Johnny was alive. This is why he tells Randy his intentions of telling the judge that he is the one responsible for Bob's death.