Sumin probably would still go back to Seju at some point later, or she might never ever will. In real life, not everyone would go back to their ex. The scene where Sumin was blushing from looking at the girl who was selling her incenses could imply that she wasn't broken so much after her breakup with Seju. In other words, Sumin would be able to move on like other people too. Moleguy's involvement made Sumin coming back to Seju happen imo. Should moleguy have done nothing at all and waited for Sumin to go back to Seju on her own instead? No, from his perspective. Why should he have to wait while he didn't even know how long it would take, and would the day even come? And how could he not want to do anything while seeing that Seju couldn't even have her strength to talk back to him? Also, how long would Seju be able to wait for Sumin? What if she killed herself before Sumin could break up with others, get drunk, miss their old days, and finally decided to come back to her?
Not everyone comes back to their ex but well they called themselves fated with each other, grew up together, first love and all that jazz. If she could be that easy to move on, I believe she shouldn't have revive her moments with Seju every time she got on with a girl and compare them to Seju. And with that red head, we could see Sumin seems to be happy, going buy her necklace and all, just like before the accident. I mean, you could happily date another girl but it doesn't mean you already forget your past love. But yeah, you have a point, we cant know for sure if she would go back or not.
I don't think that guy can know for sure Sumin will go back to Seju for revenge after his torture, though. He's no psychic, just like we cant know for sure she will go back to Seju or not. What he did will increase that hatred, that's for sure but how could he knows that Sumin will not go on a spree of dating thousands of girls just for the sake of it while carrying her hatred for Seju forever and disappear from her life forever?
I think, what the man did, it's not really about impacting it to Sumin, it's about impacting Seju herself, note that before that accident, she no longer wanted to eat or have the will of living but after that accident, she actually blamed herself, Sumin was like that was because of her family, so out of guilt, it's leading to her promise to wait for Sumin till she dies, and she actually stopped her terrible way of living after breaking up and actually try to be alive, just to wait for Sumin. I believe what that man did was to keep Seju alive for Sumin, not for Sumin to take revenge on Seju, aka, he wants to give Seju a reason to live, and for that he also granted Sumin a way to revenge Seju
Perhaps, the fact that Seju cheated on Sumin with a GUY had ruined their relationship forever already. Moleguy hurting Sumin would be able to force Sumin to be back by Seju's side again for her revenge. Living together can increase a chance of you falling in love with your ex again too. If his method had not worked, at least, being hurt like that could make Sumin more damaged; being more damaged diminishes a chance of her moving on with other. Yeah, the damaged Sumin might have end up dating more women, but this would not eliminate the possibility of her, at some point in life, missing her first love and go back to Seju again either because nothing would be able to change the fact that Seju and her did grow up together, they're each other's first love, etc. I don't agree with moleguy's way, but I don't see how not doing anything at all to Sumin was a better choice. Like right now that she is moving on with Sungji, there still is a chance of her breaking up with Sungji and go back to Seju again on her own too, but the chance is very low, right? Making Sungji disappear forever, or using some sort of dirty trick on Sumin again would increase the chance imo (how unlikable of me. lol)
Personally, I think what moleguy did was to give Seju a reason to live, to be forgiven, and to be loved again too, but for that he had to grant Sumin a chance to get revenge on Seju. I think feeling guilty for hurting your ex can make you feel responsible and want to compensate for your guilt, and you might end up falling in love with your ex again. I honestly find it weird that after Sumin felt guilty for her 10 years of revenging on Seju, she decided to leave Seju.