Can anyone tell...why did that 2nd guy , who slapped him...kissed him !?!? Who is he?

SPOILERRRR

they kinda have that toxic relationship. but dont worry bcs our mc doesn't like him. the 2nd guy, haseong, is toxic like that. he doesn't like that our mc broke up with him and he wants our mc back. so he's now trying to blackmail him. haseong is a delinquent who didn't like that our mc broke free from that toxic environment and living happily without him so now he's trying everything to get him back in his life.

Crazy....with that type of behavior.... (╬￣皿￣)凸