I'm a little  bit confused on this matter. At first I thought Im Hyungtaek and the mom who was killed are his parents, but then when Seoktae came to visit the mom at the hospital, he said Hwayi seeing monsters must've been a trait he got from him. 

I think hwayi’s dad is seoktae... because in the past seoktae purposely rape hwayi’s mom

 funnysimba:

I think hwayi’s dad is seoktae... because in the past seoktae purposely rape hwayi’s mom

My view exactly.   Seoktae raped hwayi's mother ( In Hyungtaek's wife)  when they were young because he hated everything about  In Hyungtaek. She must have had a baby.      

I belive that Seok Tae is his parent(bc he raped the girl that was in love with HyungTaek that  can be the HT actual wife).

the woman seok tae raped was the captive woman in the house with the chains in her feet. that is now hwayi's biological mother. his biological mother is the one that seok tae killed in the hospital and his biological father is hyung taek.