i headcanon that in the "everybody lives " AU that Seungjae becomes a lawyer and eventually gets a job in the field of politics.
So what if: Hyeonseong sees him living a successful life and, wanting some justice to be done, decides to threaten him, saying he will ruin Seungjae's career by releasing evidence of his crimes from when he was a teen...
I think Hyeonseong would do this because he'd want to for once have the upper hand, to be able to ruin his Seungjae's life like he did to his. To feel in control, feel brave, do something right.
All the things he couldn't say at the teachers office.
He'd release the evidence eventually, regardless, but I see him using this opportunity to help Chae-eun and his own family in any way, so perhaps he'd ask for money, or something.
I'm sure Seungjae would pissed, but money is easier than strangulation and stabbing so... And Hyeonseong's life looks pathetic to him, he'd assume that money is really all that he wants.
Like. "Surely Hyeonseong wouldn't be that stupid as to ruin HIS own life trying to get revenge, right?" is what he'd think.