In the novel it is basically said that it’s kind of instinctual for the demons to procreate (which makes sense when you think about how hard it is for them to do so in the first place). So his dad felt the instinct to keep his son alive, because that was his job as a demon, but he hated him because he killed his wife (even though that is what she was initially for, as we have seen swans aren’t always loved). So their demonic line is guaranteed (because Roth would never kill his child, even if only by instinct, though I could go on by why I think he wouldn’t do that anyways), and the guy decides to make Roth suffer like he did (and was presumably hoping to continue the cycle with Svanhild, but Roth, though apathetic towards him, did not blame his kid).
Was it ever explain why his father wants him to suffer so much? All I could piece together is that his mother died from birthing him but it was his father's choice to impregnate his wife. Why make him suffer? Why not ignore him?