What are the chances of this ending well for all of them? hAHAHh.
Sumin was /really/ immature back then and her actions of course came with a bunch of consequences. She didn't know how to deal with her family's death. And to fill the hole in her heart, she tried to find someone to blame because of course that's easier to accept in cases like that. Part of the way of why Seju is like that right now (possessive af over her) is because Sumin herself. I think most of Seju's actual behavior is merely a reflection of Sumin's past actions.
But I bet that Sumin is not aware of that (or maybe she is? Because it really looks like they both know there's no /romantic love/ anymore but there's still /something/). The thing is that Sumin got someone to help her now and Seju didn't. While Sumin tried to heal herself over the years with going out with a bunch of woman, looks like Seju kept "waiting" because (in her mind) Sumin is the only one that can heal her, but that's not the same for Sumin.
Would be really great if they all sat down and discussed about their unresolved stuff and keep things clear so they both can move on, maybe? haHAhh. But this is a korean thing so i really don't expect a happy end.