Rumple sees it as part of his identity. He’s been the dark one for so long that it’s literally become a part of him. That is who he is. I don’t think he’s necessarily telling Belle that she has to accept he’s going to go out murdering the peasants, but he’s essentially saying “I am the dark one and you need to accept that if this relationship is going to work”
But can you (well, Rumple) separate those two: the Dark One and the Murdering? He hasn’t been able to in…well, a long time.
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"He was a lot of things to me" "The only conclusion was love"