Question

Why did Ponyboy try to assume responsibility for the stabbing death of Bobby?

Answer 1

The reason is because ponyboy experiencing Post traumatic stress disorder.
In his statement, ponyboy keep repeating that 'Bobby is not dead yer, he is still alive'. He is shaken mentally. It is a defense mechanism that his mind created because Ponyboy unable to accept the fact that Bobby has died right in front of him.

Answer 2

He didn't want Johnny to get into trou