User blog comment:Nightband/Season Seven Plot Hole?/@comment-2059966-20120527150607

I think it was a mistake the writers made (they are human after all) but if you wanna explain it in-universe it could lead to something being revealed about Purgotory or the Leviathans in Season 8.

But if everything does have a soul including the Leviathans then there is no REAL way of killing them as Death said a soul can't be destroyed by him (probably only by God as he created them)

But my oppinion is that its just a writers mistake.