I honestly feel like he must have had to kill his dad from his dad turning into one of the undead or from his dad maybe even just losing his mind. The way the teacher was asleep but then became conscious trying to stab Guwon in the previous chapter feels like a repressed traumatic memory causing trauma responses to danger.
Honestly, he feels like a trained soldier with the pulling a knife on Guwon while still being asleep. Could be related to his father, or maybe not. I don't buy that he's just a teacher, maybe that was a cover up for something (a mission for example) or it was him trying to start fresh from his old job where he had to dirty his hands or something like that...that unstable memory could be anything honestly. I'm so curious!
Teach's past seems interesting: The fact that he has flashbacks and has a spotty memory plus when Guwon found him he had a bandaged head? Bro went through something fr.