They had a child murder someone. There has been no evidence that holding the dark one dagger makes that person dark. Bae was a child and he murdered someone even if he was a bad someone. They had him murder someone with dark magic which he did not like. Think about how he reacted when Rumpel murdered all those guards right after he became the dark one. Think about how he was reacting earlier in this very episode. This entire scene was ridiculous and so out of character.
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