If I remember again, his father going out looking for water to give to teach because he's already weak..but the moment he's back he meet with guwon and at that time teach is unconscious..if you remember again, guwon will killing anyone he meet outside that not give any benefit to him even though they're no threat to him..teach don't remember it until he find his father corpses and guwon admit that he kill teach father because he's not give any benefit..teach almost get kill if he's not a teacher..
For a moment I think teach join rebellion is understandable.. he thinks guwon will not be tolerate if he ask him to stop killing people because he already see enough guwon behavior killing father that stealing medicine for his sick son and the his father death..but I don't know he should try to talk first before taking this decision because guwon right, they will not able survive without him.. ( ̄へ ̄)

Maybe I need to reread but I’m not quite understanding why MC is so upset about ML “killing” his father when it seems that MC’s parents didn’t treat him very well in the first place