This was never on the healthy side. It wasn't even on the relationship side. It is more like on the "make the best of a super shitty situation" side.
I also am surprised that people thought Seungho wasn't going to be Seungho anymore. I always thought that when he realized that he cared for Na-Kyum that could bring out the worst in him, but it is a mercy that his possessive side, so far, has stopped him from going "full Seungho."
Notice how the insane eyes came back as he continue to let his thoughts fester around the reasons why Na-Kyum was so compliant.
The man has lived in that state for 10 years, he will not change (if he does) overnight or because he caught feelings for a low born.
I'm so sorry but we're at Chapter 54, we knew this is not the healthiest relationship since 30 chapters ago, why are people surprised Seungho is acting like a douche canoe lol.