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I think Klaus regressing back to his original nature had a lot to do with the number of situations that kept getting out of hand for him in season 5. And those purist freaks literally mutilated Hayley and binded her wolf side. Klaus is the only one who can understand the excruciating pain that comes with it. It's justified that all he wanted was mass slaughter at that point. The amount of blood of Hayley's he had to see smeared in places(the coffin, the house) must have made him go insane with rage. (At that point, I was literally all in for a full-on massacre cause Hayley was nothing if not just and kind towards all factions)
As for Camilie and Caroline, yes, he had some amount of concern for them, but they had no sway over Klaus's actions or nature at all. Nowhere near Hayley did. Yup, the people who say that are both stupid and blind.
If you think Camille or worse, Caroline contributed to Klaus's character development you're stupid because he kinda had none