Responses
I took it that Keith realized he couldn't mark Yeonwoo and make him only his. Honestly, this manga is giving me more anxiety than anything. Like damn, Keith. Just admit you love Yeonwoo then Yeonwoo will do the same. I think that's how it'll go.
The way keith said that "he can't mark anyone" means that this is just a situation that is totally inconvenient for him... I don't feel pity nor empathy for him. The author want to shove this 180° personality change in my throat but sorry this is not going to work. I can't simply forget nor forgive what he did to yeonwoo.
Just a Speculation:
BTW if yeonwoo die in the future... May the yaoi haven take him... Which means that more suffering for Keith.