it's because he knows he was wrong lol, he knows that he was acting like an asshole, and that nakyum wasn't trying to runaway, and also realizing the negative affect he's having on nakyum, because he does have feelings for him.
He only knows he was wrong because Kim scolded last chapter. If he realised the negative effect he has on Nakyum, he would have left him with his sister.
that's what someone in their right mind would do. you can't expect him to do a full 180 because of how unstable he actually is, but you can see him start to realize the weight of his actions. he doesn't know how to express his feelings for nakyum, because all he knows is violence and control.
He heard the whole conversation with the sister btw. I wish the author would have shown more conflictuality in both the characters.
ok what’s this sudden change of attitude from seungho ? I’m very worried for nakyum’s noona