so, the hypothesis that _Zodiac =Monster of Florence_ would make a lot of sense, because it explains Douglas's observation. if that was the case, the Zodiac would be an absolute exception in the sense that he grew old and retired.

further, Douglas, from many years of experience of investigating serial killers, observes that killers have a 'signature'. a handwriting in their crimes, so to speak. and at first glance, the signature of the Zodiac and the Monster are very similar indeed, as others have mentioned.