The way I see it, MC intended to hurt Tanja, to scar her emotionally as she scared them. He didn't figure on not just hurting her but breaking her. One he realized he broke her, he felt he had some responsibility, an opportunity to reshape her. Yes, initially, he had two branches to reshaping, but over time, I realized that the pet route was not sustainable given my MC's personality and history with dealing with broken and damaged women.
Personally, I could have accepted him walking away and letting someone else rebuild her, but without his intervention, the chances of her coming back even worse was a real possibility.
That's my take on the story, given the choices we had.