Basically Taehwan and Sooyoung started as friends and eventually developed a romantic relationship but Taehwan is crazy so he went out of his way to get Sooyoung ostracized at school in order to alienate him. He also sabotaged Sooyoung’s college admissions. He did all of that so that Sooyoung would on rely on him because he wants Sooyoung all to himself. This is a summary of the most important stuff.
So ml beated the shit Outta of mc's friend and he made mc isolated, so that he could be the only one for mc ( they were in love) and then somehow mc finds about it and then he gets mad/ afraid of ml and then ml threatens mc that if he leaves he will kill himself but mc says " whatever" and leaves the ml . So basically that's it.
Man I would have run too