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.
The real parents are definitely In Hyungtaek and his wife. This is evident by the posters and decoration kept in the top part of their house, and the heart-wrenching scene when Hwayi found his mom (the one in the hospital) got murdered. Seoktae said that he got the trait from him isn't biological, rather raised, or simply relatable. Seoktae has experienced that condition, which is caused by PTSD, and he thinks he could therefore 'cure' Hwayi of it.