“During the original Zodiac investigation, detectives explored the mind and methods of the killer, and sought the advice of a psychiatrist at the California Medical facility in Vacaville. After examining the writings of the Zodiac, the psychiatrist concluded, "He is probably a guy who broods about cutoff feelings, about being cut off from his fellow man." Vallejo Police Captain Wade Bird speculated, I think he’ll prove to be a genius who got so far out he went over the edge... I don’t believe a man this disturbed could hold down a steady, regular job. He’s too far gone for that.” Anonymous. (2007) UNSUB: The Psychological Portraits. Retrieved from http:// http://www.zodiackillerfacts.com/PsychPortraits.htm
Furthermore, there is some motivation theories that some psychologist found. Robert Morton argued that “as the identity of the Zodiac is still unknown, one can only guess (using clues from his letter, his MO etc.) at his motivations. In his own words (from the only cipher to be solved), to kill gives me the most thrilling experience. It is even better than getting your rock off with a girl”