Because Sarang needs money. The game has a bet of 50,000 won everytime a participant refuse to do it.
Her batshit of a father took all her money just so he would leave her alone. Which means despite Sarang being diagnosed having mental issues, she still holds back from killing her father despite wanting to do so.
Sarang has no concept of empathy or humiliation. Also it was Subin who said first that whoever refuse the dare gets slapped hard, and Sarang just followed the rule. It's not her fault that it backfired to Subin. Subin and her friends are already aware of Sarang's weirdness, yet they still keep dropping stones on their own feet.
Subin dig her own grave, and she has many chances to stop when she was confronted her games are not fun anymore. Yet because she's a wannabe psycho and wants to prove she's special, when seeing the true representation of craziness(Sarang) she gets undermined and it hurts her ego. Not to mention, her own so called 'friends' reminding her every single time that if she's not crazy, she's just a nobody.
Also I agree with the view towards the student council president. I think he's the representation of those goody two shoes that only acts on their benefit and for their image. He said it himself that universities in the career of prosecutors not only check the grades and score of entrance exam, they also check the background and deeds of the student.
Also about the kiss thing in the earlier chapters Subin literally dragged the orange haired guy and blackmailed him to tell Sarang he's uncomfortable of the kiss thing. Subin got super jealous that the orange hair guy has a crush on Sarang and was gonna spread a rumor about the 'fantasies' he had about Sarang.
Also Sarang already said she was not participating, but it was Subin who keep insisting and when blackmail about the kiss did not work because Sarang was helped by the orange haired boy, Subin switched tactics with money.
That's why, Sarang is indeed the victim here. Subin is the one trampling on others to cement her reputation as a psycho crazy girl. But unfortunately for her, she targeted the real diagnosed psycho. That's why her image and reputation crumbled slowly every time she do an embarrasment game with Sarang. Because Sarang has an unpredictable set of thinking it always throws everyone of guard.
Most of all, I feel so sorry for Sarang. In the last chapter she's clearly struggling now and her dangerous intrusive thoughts are showing because she's ran out of her meds. Her having a symptoms of spiral were showed in the panels so I don't know how you still feel sorry for Subin.
Subin is a wannabe, Sarang has an illness and I think anyone could guess who's the more pitiful here.
Yeah in terms of her friends, they reinforced her feelings of 'distinction.' Now we don't know what grade they're in but Its cool how accurate they portray 'edgy' teen phase lol. Back to topic, but once this image gets challenged she tries to do everythibg it takes to validate this identity. Its really cool, cause it presents accurate 'static identity' or 'themed identity' are. (I haven't done research paper abt this topic totally would come back here *behavioral student here* to update)
So far yeah diagnosed psychopath is so cool to see in fiction that actually aligns to symptoms (though I need to study it further) tottallyyy so cool.
Subin IS pretty bad. she doesn't know the boundaries at the moment. she'll keep pushing and pushing others (in this case, Sarang/MC) just so she could come out as the more 'weirder' one, the more 'unique' one between them. it's understandable why she'd want to feel that way but that does NOT mean she's not bullying Sarang.
you can feel bad for a character and the way they think, but you shouldn't erase the one thing that makes that character the way they are. Subin IS a bully, even without Sarang in the picture. I mean, she made the orange haired guy stop going to art class right after exploiting his presence that gave her money :)
i get that subin has her insecurity problem but that does NOT tone how much she did to sarang. just because sarang is okay on what subin did to her doesn't make it right. imagine if sarang is actually a "normal" girl and they kept doing it plus poking it for fun, one might actually killed themselves over that. so does imagining sarang is "normal" justified than when shes not "normal" now?
also there's this thing about social status, the one crucial thing about them all is about social status. that orange head don't want to admit he likes sarang because sarang is a "weirdo" and people have a various opinion about her, so he don't want to be labelled as a loser, that would make him friendless despite him actually liking her.
the council president also taking care of his social status, making him look prestigious and actually have a good record. remember that the guy wanted to have a good record? he's honestly also twisted in his own ways, he wanted to expose what that friend group did to sarang, hence maybe thats why hes loitering it.
then about subin, she's definetely the biggest case of these. she's been ironically called a weirdo/pyscho because what she did in the past was so blunt than what normal people do in social spaces, some call it brave, some call it rude. but she wasnt familiar with all of that treatment because she thinks that its "normal" to be like doing all that, to what's she's currently doing. honestly? she's actually more similar to sarang than anybody. the only difference is subin actually feeding into these compliments, thinking being weird is actually a compliment... but when in actuality being the so called "weird & a pshyco" is actually harder feeling it than you didn't think youre actually weird, or just being called that you're weird.
so yeah thats that

The orange haired guy said himself he’s not bothered by the kiss and the MC also knows the student council president so maybe he could make sure nothing escalates to the committee.
Subin isn’t that bad tho. MC is the one who literally attacked her by slapping her so much. It’s so weird how after that incident, people thought MC was the one who was bullied. I do kinda feel bad for Subin. She feels this prank stuff is her whole identity and she used to get along with her friends doing this. Now the joy is gone and Subin is getting nowhere daring MC to do smth. The student council president is weird. Idk why he’s not actually making an effort to make the pranks stop. He just lets them go on and says do it to me and acts like boohoo I hate u Subin. If he had a brain, as a student council president, he’d make sure it stops instead of entertaining it