Well, in supercheesegirl's post, the brief sentence "Dumbledore died" is actually given as an example of an intransitive sentence: that is, a sentence that can't be put into the passive voice. The author notes that "You can’t make an intransitive sentence passive because there’s no object to flip."

"Dumbledore died by zombies" is not really a valid English sentence because die is an intransitive verb. The usual way to express that idea would be to use a transitive verb such as kill, which can take an object. Therefore you would either have "Dumbledore was killed by zombies" or (to turn it around and make it active) "Zombies killed Dumbledore."