The Dark One is not a person; anyone on the show can become the Dark One simply by willingly killing the current embodiment of darkness. In this clip, from the first season of the show, Rumpelstiltskin murders the Dark One Zoso with the Dagger that controls him. By doing this he must bear the consequences of his actions by becoming the Dark One himself. The show regularly tells the audience there is a price to pay when using magic; just as the bible teaches that there are consequences of sin.