They explain it early on. Her mum passed away cause she was sick. But mc is keeping her in a freezer cause she will be forced to live with her dad if they find out her mum is dead. Otherwise mc would have had a funeral like normal.
Ofc mc isn't aware there may be abuse of corpse laws in korea that may incriminate her after she turn 18.
Np. While mc may have some antisocial disorder, she thankfully has not done anything wrong as far as we know. And she was doing well while she had her meds. In fact the most aggressive ppl are the people that don't have any disorders in this story. Like her dad and that annoying girl that keeps harassing her.
Maybe it's a theme in the story that mentall illness doesn't equal being violent

I’m honestly just curious how the mother dies, what the mc’s childhood like, and what’s the true face of the class pres.