Dean became a demon when Metatron killed him while he wore the Mark of Cain. Sam brought him back to humanity and, though Dean still carried the Mark, he was no longer a demon. If he had been killed again while wearing the Mark, he would have become a demon again, because the mark would not have let him die.

Dean spent a year in purgatory before being able to escape with the help of his vampire friend, Benny, whom he met in purgatory.