As recall correctly, the baron found them on the way to elope with her lover and the baron's only daughter that he seems to greatly love JUST after he came back from war. With all that ugliness and to come home only to find that? I am not surprised he was seeing red. I dont think the abuse started before he she decided to go happy ever after with someone else.
Yes the barons wife is a scumbag and a cheater, that ran away with their only daughter. No argument there. However the baron himself answers this with murder. He murders her lover before her eyes and expects his wife to come back to him afterwards. When this understandably doesn't work out like that he violently subdues her and locks her into is castle for years to come. I would say nothing against the baron just taking his daughter back with him and. That would have been resonable. However he decides to steep much lower than his wife ever did instead.
Also, how can he kidnap his own wife and daughter?
How does one kidnap ones own wife? Simple. By dragging her to your castle with force, against her will and then lock her up there against her will. Just because she is your wife, doesn't mean you can do whatever the hell you want to her, even against her explicit wishes.
He was trying mend things it seem and she just mentally and physically abuses him that in the end, she got what she gave. That she got it from the same person she did it to shouldn't matter because he's a he.
I think you fail to see the point. The Baron killed the man she really love before her eyes, he keeps her prisoner against her will and he most likely rapes her. That he tried to mend things while continuing to hold her prisoner means little. Her lashing out against him isn't her being abusive. That is just a normal human reaction to being held prisoner against your will by a murderer that killed a person you loved. And no, the fact that he is a he does indeed not matter in the sligthest. If the roles were reversed and his wife were the one to kill her husbands mistress and then lock him up in the castle against his will, she would be the more despicable one. But that isn't the case.
Never dish out what you can't take. As my dad always say.
Words of wisdom, even if I prefer the saying: "Don't kidnap people against their will and then expect them to love you."