I can see it going many ways, but I really don't think Moon Young suffers from antisocial disorder. Sometimes this summaries are very misleading, to me this is the case here. From what I gathered she could be suffering from PTS caused by childhood abuse which could also trigger many unbalances in her relationship skills and behavior. To me, her parents seem very ill, her mom seemed very possessive towards her so I think she was mainly kept from developing early childhood friendships. It's also very odd to me that her dad would literally build a house in the middle of nowhere. Regarding her mom, I don't think she killed her, but I do think she locked her in after finding her dead. Maybe her dad also killed her mom? Or maybe she killed herself? I don't know, but we'll keep getting clues. But if it were her dad it would also give him a reason to try to kill his own daughter. He could be having an episode and convince himself that it was all her fault... I don't know, but it sounds like a possibility to me.
Regarding the butterfly and Kang Tae running scene, I thought about maybe she knowing about his brother thinking it was a butterfly that killed his mom. Maybe she killed all the butterflies trying to show some kind of protection? It's rather easy to think that that was her being mentally ill as a child, but when children are not shown regular healthy love and care in early ages they have a different language and I could see that being the case here.