
It was his bias attitude towards the Mc (thinking he slept his way through) and treating as such and him just being on the sidelines when the mc was clearly being bullied .so he was bystander when he could’ve done something which he did fix later on the story but that’s after he realize that he actually liked the mc a lot . also the fact that he kept the secret
So summary was that he was biased ,bystander ,and a coward but this all before he realize and confessed but people could get second chances I don’t think he did anything unforgivable he just messed up
OH I COMPLETELY FORGOT but he’s taking advantage of the mc because he knows he’s too nice even when he’s angry which I think is the main problem with others
Also the mc deserves to angry and have a time to be angry it’s not like he’ll be mad forever

Additionally, yuan being a big influence on his classmates and being rude on mu who is a transferee caused everyone around mu to despise him. That's why mu moves away from yuan on the initial chapters and became a loner just so he wouldnt be bullied by yuan's friends more.
Not to mention that yuan hid his identity from mu. And mu having bullying experiences in school because of how yuan initially treated him is a betrayal for his part when he learned that his only comfort behind the scenes is yuan himself.
I really just wanted yuan to give mu some time alone.

he was a bystander and an enabler. his influence to his classmates had big impacts to how the classmate bullied uke.
He blatantly showed disgust in first chapters, alienating the uke
upon knowing how uke felt about his classmates he continued to not do anything knowing he could easily influence the classmates behavior
i could go on lol. seme was a literal definition of a manipulative sad boi but he did reflect on his actions
I dont get it why is everyone mad at the seme...i dont see wht he did so wrong..its not like he bullied him he just didnt talk to him...and he soin talked to him more later..i really sont see wht he did so wrong i personally like him