I can't get myself to read it at all but I am curious Can anyone tell me why did he beco...

MAJOR SPOILERS:

The ml and the mc (in-seob) went to the same high school in America, and the mc's friend, jennie, had a crush on the ml and was a bit delusional and thought he also liked her bc someone was pretending to be the ml and responding to her love letters. She convinced the mc to attend a party to prove that the ml does love her since mc didn't believe it, but once the realization hit that ml wasn't the one responding to the love letters and it was al a ploy of bullies (ml's friend group), mc became fed up with jennie for being delusional and humiliating them, so he started ignoring her.
Bc jennie lost her only friend+ had a troubled home life + had bpd, she ended up taking her own life and her heart was given to the mc who had to get a heart transplant. So the mc averts the blame onto the ml for jennie's death and tries to ruin his career and reputation just like how the ml did to her. Although it is revealed later down the line that the ml was unaware of what his friend group was doing and lowkey didn't even know jennie existed.

lmk if i need to clarify anything

So basically In-Seob (manager) was close friends with a girl named Jenny who was being bullied, had BPD, and an abusive mother. Jenny was OBSESSED with Woo-Yeon (actor). She believed that they were destined for each other. So she had In-Seob help her write him love letters in Korean.
Unfortunately, some mean girls intercepted her letters and wrote fake love letters back. They then invited her to a party to make fun of her.
In-Seob tries to warn Jenny that the letters she’s been receiving are probably fake but that made her mad and called In-Seob a bad friend. Even so, they both went to the party where Jenny inevitably got humiliated in front of Woo-Yeon.
Jenny and In-Seob get into an argument bc he was like “I don’t wanna say I told you so buttttt…” And then she killed herself and In-Seob believes it was his fault for not being a good enough friend. Jenny left a letter behind before she died blaming Woo-Yeon for her pain and the only way she can be in peace is if In-Seob gets revenge or smth like that.

TLDR; manager dude had a friend who was in love w the actor in high school, she gets humiliated and heartbroken and then dies. The manager then goes on a mission to avenge her death by ruining her first-love’s life.