When playing agains the vampire, it states that when the vampire kills a town elder it becomes an evil elder and joins the vampire. Now does this apply only during a Showdown or also because of a Mystery card which kills a town elder?
If the mystery card indicates the Boss monster is the one who killed the elder, then the effect takes place.
There is one card (with multiple copies) in particular that does this, though I can't recall what it is called. "Murder" maybe? Anyway most (all?) boss monsters have a special rule regarding this card.