The Son of Sam Killer

David Berkowitz, aka the Son of Sam, plead guilty to eight separate attacks that terrorized New York City in the hot summer of 1976. His weapon of choice was a .44 caliber revolver which he used to kill the six and wound seven others. The attacks put the entire city on alert and Berkowitz managed to elude New York's finest for over a month as he taunted them with letters. After his arrest, he confessed to the crimes but maintained he was under orders from a demon named "Sam".
He would later admit that this excuse was indeed a hoax but the questions remained.... What possessed David Berkowitz to commit these atrocities?