Considering its Seungho we are talking about its a mix of both. In his eyes he never does anything wrong. He is simply showing kindness but Nakyum a lowborn be getting brave and lashing at a noble. I mean it happened before after Seungho raped Nakyum and Nakyum was having a bath. Seungho asked him why he was still crying, brushed his tears away and Nakyum swatted his hand away which pissed Seungho off. And he said something along the lines of "dont get cocky just because ive shown you a bit of kindness"
Even tho he was worried what I think is that seungho doesn’t really know how to deal with na kyum crying/ being emotional so I think he did it his own way and slapped him so he can snap out off it, even though it may not be the best way to deal with the issue. And I think over time he’s going to get better and treat na kyum better.

This author is pretty good at character development/consistency because:
1. We can see that Seungho cares for Na Kyum, not just his body but also his emotional well-being. Upon seeing Nakyun upset, SH has a worried look on his face, and ask after him immediately.
2. Nevertheless, Seungho is no mr gentleman. I dislike it when stories give a douchebag seme 180 degrees turn all of a sudden because of lurveeee. As Nakyum rages at Seungho, Seungho immediately reverts back to his sadistic self. For Seungho, himself is still the most important being.
So there you have it. A perfect balance between development and consistency. I look forward to the day Seungho places NK ahead of himself, and NK's mind succumbs to Stockholm Syndrome. This is no fluffy romance, y'all.