Teachers, friends, classmates, parents and relatives couldn't believe that he killed his girlfriend. Reality is sometimes very difficult to face, especially when feelings are involved. Parents will never accept that their own child, raised in comfort, love and support, could be capable of killing someone. Disturbing people may well be disguised under a charming appearance. Many serial killers were known as honest, brilliant, and admired people. When a teenager carries a burden like Adnan was, probably feeling guilty about giving it back to his parents, his religion and customs could transform him into another person for a few moments when he felt betrayed, humiliated, dishonored and abandoned by the one for whom he did all this. Then he kills Hae Min Lee in an attack of rage or probably, as Jay said, Adnan was planning the murder in detail; that's why the police couldn't find enough evidence to investigate it clearly