Article snapshot taken from Wikipedia with creative commons attribution-sharealike license.
Give it a read and then ask your questions in the chat.
We can research this topic together.
As revealed in the third season, demons were all once human, but their souls have been corrupted after spending hundreds of years in Hell, leaving them as large, black clouds that generate electromagnetic interference. This revelation, which according to series creator Eric Kripke "opened up the mythology in an interesting and complicated way", was first pitched by writer Ben Edlund. It was further expanded upon to both convey the belief of the writing staff that the "worst evil is the evil inside all of us" and to serve as a plot device by showing Dean that being in Hell will "burn away his humanity" and turn him into the "thing he hates most".
