Young Kidaichi's Murder: File 1 - S2-E1
Stupidity: Kindaichi behaves in a criminally illogical way in this fourth parter episode. The original victim told 4 separate people some sort of keyword for the party, each one linking to another person. Kindaichi after at least the second if not the first victim could and should have asked EVERYONE (he had every opportunity for that) if they received such a 'password' by Tachibana before he died. Instead, he pointlessly follows the trail of breadcrumbs and two if not three people die for nothing.
Stupidity: The master goes through all the trouble of making Luke the vessel so Luke can go and kill several people and risk getting the attention of the slayer, when all he had to do is have his minions go out and kidnap a bunch of people and bring them to him and he could've fed off them himself.