The theory that best explains Abe's change of attitude towards his action is the Cognitive Dissonance theory.
Cognitive Dissonance is the mental discomfort produced by the contradictory beliefs that a person holds. In this case, Abe suffers from cognitive dissonance as a result of copying answers from his classmate in the exam. In order to relieve from this stress, he tries to justify his action by denying his wrongdoing and transferring responsibility to his classmate by claiming he did not do enough in order to prevent him from copying.