I don't think of Seju as evil, but I do call her for being manipulative for whatever she's planning when she was pushing Sumin towards Sungji in the early chapters, and that conversation she had with Sungji in the office in chapter 13/14 (?).
About the claiming of Sumin as an object, I don't see it like that. Yes, she has this weird, unhealthy possessive streak regarding Sumin, but I can understand her point of view. Her family is horrible so she grew up emotionally warped and she clung to the first person to ever show her a bit of love, Sumin. She's always considered Sumin her family, whatever "family" means to her (familial love, romantic love, both, something else?). And now, they're even more similar to each other, one who can't love and one who's lost the ability to love. They're birds of a feather. Granted, Seju's cheating and the dude made Sumin that way. But it's small wonders Seju thinks the way she does, "Look, we're such wretched creatures that I'm the only one for you...." That's just sad.
@ jmv, yeah Sumin isn't an object to claim, but she has been returning to Seju's bed for the past decade by her own will. I do wonder about that though. She was all drop-dead/screw-your-face/you-can-die-in-a-ditch-for-all-I-care towards Seju right after the accident, but she seeks out Seju time and time again. Why? Familiarity? Because Seju was her first everything and forgetting is impossible? We'll need an explanation for that. But Sumin's actions doesn't help but further cement Seju's confidence that Sumin will eventually return to her side.
I agree that Seju is the one ultimately responsible for her own life and moving forward. I want her to for her own sake, but it's hard do so if Sumin keeps on coming back.