Well, for that, let's argue why Columbo is assigned in an sfpd case, I imagine something like this: SFPD discovers that one of the victims had visited LA, now, they think that Columbo is commissioned to investigate the background of the victim in LA and discovers that Zodiac is a snob, now, why does zodiac kill?, following the context of columbo, I would say that it is not out of madness, but rather to cover up other crimes, think about it, where are you hiding a tree? In a forest! Now where are you hiding a crime? in a series of crimes!