What is his problem with his son having an omega I didn't get it........ 
Waku Waku 
Comment by yoona on Instant Family - Chapter 33
2 Replies
@yoona I'm going to take a wild stab at this and say that his dad wanted Sejoon's dad (I don't recall if it was stated if he was omega or not) and killed the mom to get her out of the way but his plan backfired and costed Sejoon's dad his memory and health. OR, dude had the shittiest luck and ruined Sejoon's family out of jealousy and something else. There's something messed up going on and I don't trust ml's dad. Could also be that he's jealous of his son already being bonded with an omega and he never did.
@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