Teachers, friends, classmates, relatives and parents 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 child, raised with comfort, love and support, could be capable of killing someone. Creepy people could be disguised very well under a charming exterior. Many serial killers were known as respectable, brilliant and admired people. When a teenager carries a burden like Adnan was, probably feeling guilty for giving something back to his parents, his religion and customs may transform him into another person for a few moments in which he will feel betrayed, humiliated, dishonored and abandoned from the person for whom he did all this. He then kills Hae Min Lee in a fit 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