Why can a ghoul player character become an antihero, while a vampire / werewolf one cant? Because you only can become a vampire or a werewolf by your own choice, which involves several conditions including pledging allegiance to the evil Bloodfountain, but becoming a ghoul is a completely accidental (you need to catch The Plague).
The Demonic Possession background choice is followed by lines saying that the player character can't control his own actions and is forced by an outside mind. Who does control the player character? The player. Yes, the player is the demon controlling the player character.
