i think his dad died on seonyul's birthday. i know it's cliche but the dad bought a cake and he was talking with his wife on the phone before soo hyun murdered him
so... i think seungjoo's real father is that man who died on jin family's house. killer is the mother and real father was trying to expose everything. seungjoo also knows this. mother is also the killer of jin isoo's mother but this plot is too predictable, maybe they want us to think it this way