@OpalEyes
good guess but actually that is not what happened.
SPOILER ALERT!!
Taeheon's dad is not his actual dad, he only took him in because he wanted to use taeheon to spy on sejun's family. Taeheon's foster dad cause sejun's family into accident..it had something to do with money. After the accident teaheon felt bad and left sejun to work on his own revenge plan