User blog:Jartsa67/question about souls

ok so, when sam and dean met death, he told the brothers that souls are pure energy and that even death himself cant destroy souls. but when lucifer was banished from heaven, lucifer twisted the human soul to despise god, and over the years, that bent soul became the first demon known as lilith. so demons are basicly souls too, they dont transform into something different when souls are tortured in hell. so if demons are souls, and not even death can destroy them, how the demon-killing knife kills the demons? when human being possessed by a demon, and the demon gets killed via the demon killing knife, does the knife kill or destroy the soul of the possessed human too? what happens to the "soul" of the demon when it is being killed? does the demons soul somehow get "purified" and maybe the soul goes to heaven then. what do you think?