The Zodiac Killer starts his letters with, "Dear Editor", before introducing himself as "murderer" and "killer", and
confessing to having committed terrible crimes. Even if the Zodiac would have considered his misdeeds as justified, his
selection of the words "murderer" and "killer" clearly indicate that he had the capacity to understand the wrongfulness of
his actions. Consequently, the Zodiac's words suggest that most probably he was, by definition, a legally sane person at the
time the letters were authored.
The Zodiac calls his teenage victims "teenagers", while calling his 22 year old victim, Darlene Ferrin, married mother with one child, "the girl". Obviously, the Zodiac knew that Darlene Ferrin was not a teenager anymore. If not already before or at the time of the crimes, the Zodiac must have known this from the news that followed. In any case, Darlene Ferrin did fit his target victim profile, either at the time of the actual crime already, or confirmed by the Zodiac with his retrospective acknowledgement through his July 31, 1969 letters. Consequently, his target victims were not only teenagers. If at all, marital status and motherhood were not priority criteria for the Zodiac to select, acknowledge or count his victims.
Throughout the letters, the Zodiac Killer refuses to recognize and accept his victims as individuals through avoidance of their names. He depersonalizes the victims by just calling them "teenagers", "boy" or "girl". The names and histories of his victims must have been known to him, the latest from the newspaper articles that followed his crimes. Furthermore, the Zodiac does not mention his surviving victim, Mike Renault Mageau. Possible reasons for not mentioning Mike Mageau could be that the Zodiac only counted definite murder victims, or because he considered Mike Mageau's survival as his failure to kill, or a combination of both. Additionally, the mentioning of a surviving victim would have weakened the "Killer" and "murderer" image that he was about to proclaim.
Obviously, the Zodiac Killer found it necessary to authenticate himself as the real killer through provision of a list of specific crime details that had not been released to the public at that time. Most probably, the July 31, 1969 letters where not the first letters of the Zodiac Killer, and previous letters might have been ignored or considered a hoax, or the Zodiac lived in an environment where it was common that proof was needed to support any claim, such as for instance a mistrusting family, or an academic or even police environment. Another possibility could be that the Zodiac read about other serial killers that sent letters to police and/or media, such as for instance Jack The Ripper, and followed their example.
The Zodiac Killer's use of the words "I shall" is remarkable, with the popular Zodiac-British Connection theory just being one of the many possibilities. We find the use of "I shall" as poetic or archaic/religious form, or as form for particular emphasis, or as indication for another Jack The Ripper inspiration as at least same plausible. There is frequent use of "I shall" in the Bible, and Jack The Ripper used "I shall" in his communications. The latter might be even an indirect reason why people have been seeing a possible Zodiac-British Connection. At least same remarkable is the Zodiac Killer's consistent spelling of the word "Christmas" as "Christmass". If the spelling was intentional, and we very much believe that it was, then it would strengthen the before mentioned possibility of an old English and/or religious connection.