I looked at the raws and from what I can tell it's his twisted way of protecting/taking care & his fascination with her, while honoring his master's wishes. I don't wanna spoil too much but I think his thinking is if she has his child/heir no one can ever mistreat her. Therefore she will be secure for the rest of her life.
Ohh
Can u give me the raw link pls?
There are some more links in the earlier chats but here's one:
https://lectormanga.com/library/manhwa/49105/noche-de-inmoralidad
Why the hell did he rape her and treat her so horribly if he knew her true identity?