But once Hook found out what she had done, he was prepared to literally send her and her entire family to hell because he was angry at her. If Emma had enough of herself within her to know what she was doing, Hook did as well.
I just saw it as an addiction. When you give an addict a drug, they are going to abuse it. Emma wasn’t a bad person before. So when she got the DO power, it didn’t have the same effect as it did on Hook. Hook had been trying to redeem himself and be a better person and the darkness brought out his thirst for revenge and hatred. But just because you give a drug addict a drug and they abuse it, doesn’t mean he’s a bad person. Especially since he was able to fight it in the end and not go through with sending everyone to Hell.