Borderline Personality Disorder.
I think what we know of his history clearly points to Schizoid Personality Disorder possibly progressing into Delayed Adolescent-Onset Schizophrenia. Borderline Personality Disorder, OTOH, would ordinarily involve a lengthy history of stormy interpersonal conflicts and contacts with social agencies and/or law enforcement with impulsive acts (this episode seemed to be anything but impulsive as it was planned for months). This guy was just a “different” sort: a loner. Obviously the act itself involves sociopathic behavior. Whatever his correct diagnosis I do believe his ability to understand right and wrong was intact, although possibly distorted. His telephone message seems just like one a guy I knew in high school and college would make; that guy committed suicide after becoming increasingly more bizarre and being diagnosed as schizophrenic. He would do things like come and bang on my dorm room door late at night for no reason and giggling like The Joker.