Okay, I feel like I need to make this clear:

If being a hexenbiest just in general causes people to have no moral compass, to be violent and sadistic with no control over their actions, then both Juliette and Adalind have done bad things that they would never have done if completely in control of themselves.

(Although I think it’s important to note that Adalind did a lot of horrible things during the time when she was without her powers as well, so I’m not sure we know for certain what she would or wouldn’t do or her own free will.)

But it doesn’t make the people they’ve hurt less hurt, less angry. And it doesn’t make them any less likely to associate that pain with the person who did the hurting. Which means Nick having a relationship with Adalind would be massively unhealthy and bad for him, and probably bad for her, too.

Just like Nick having any relationship with Juliette in her current state would be bad for them both–and non-consensual, since Juliette is programmed to take orders, not make her own choices.