I don't think he's mentally ill, but he thinks differently from other people so he may be neural-atypical or autistic? Mental illness is debilitating but that doesn't seem to be the case here.
Yes he may not and just a complete out of touch person but the possibility of him being in a spectrum is high. I know someone like this.
They think a calculation is a must. Calculation and Analyzing for everything, everywhere and for everyone. I thought at first he's insane or a psycho but then he was just mentally ill, in a OCD branch to be precise.
But yeah maybe you're right.
I think Hyunmin is also have mental health issues. Relying to games for his own environment and analyzing emetions and everything. Treating relationship as some code that needs to decode. Hmmm. People mostly like this are mostly in a spectrum and high chance of mental illness.
Ok! Put your knives and guns down, your pitchforks and torches, and everything that makes everyone want to execute me.
I'm not saying his actions are forgivable, neither do I say it's justified because of mental illness. I'm just pointing out that most of his actions are not because he's awful like Pilwon (if you know, you know) BUT MAYBE! he's just mentally ill.