
I hope his Ex confess what that crazy psycho did and I hope this ex won't do something that'll harm the mc

It was in the subtext. Gyujin asked Sooyoung if he liked him. Sooyoung even he didn't like him, just out of curiosity he said yes. Here he accepts he dated Gyujin, but it ended in regret.
https://www.mangago.zone/read-manga/the_edge_of_ambiguity/uu/br_chapter-141696/pg-19/
Soyoung accepts he said yes, "I like you" to Gyujin just out of curiosity.
https://www.mangago.zone/read-manga/the_edge_of_ambiguity/uu/mk_chapter-6/pg-4/
The writer is great at portraying Sooyung. He is really insecure at coming out with his sexuality. He always backs off due to the trauma that Gyujin put on him. Jihwan knows he was in a relationship with Gyujin, but Sooyung doesn't want him to find out. So when Jihwan asked him if he was in a relationship, he said no, but his mind says otherwise.
https://www.mangago.zone/read-manga/the_edge_of_ambiguity/uu/br_chapter-159811/pg-8/
So they dated, but Sooyung due to his trauma, wants to forget his first relationship and he is in the right to erase that part of his life and say whatever he wants.
Sooyung will probably end dating a girl after all this ordeal.

It will be weird he date a girl. He is attracted to men after all. The more realistic thing to happen is that he will never let himself to love someone else again. The poor guy have two sociopath for first experiences... And it's likely he will still attract more in the future. His "best friend" did say he always altracted weird guys. But it's a manhwa, so ML still got his shoot to have a "happy ending" (can we really call that happy ? ^^') with MC.
WELCOME BACK, KARYAN!!! I MISSED YOU SO MUCH!!!