We actually don't know if Guwon killed his dad, in the first chap the dad was already dead by the time Guwon arrived also no even the teacher is 100%, he believes is that way do to the thing that happen with the guy that got killed and what doc and shady lady told him.
In his believes Guwon is a psycho monster (prob true) so anyone linked to him can get killed or Guwon will kill it
Aggred and another point. The ML is not Negan. The ML seriously didn't even know he had a whole body part before teacher actually told him (his peepee). He didn't have a childhood (to be seen) and he doesn't even know the meaning of how many things as words , how society works etc ? A heck ton. He is not stupid , sure but eh I wouldn't call him a monster for going for the most straightforward,basic (as in point à to b) ,blunt logic in a post apocalyptic world.

I guess it’s funny to see how most comments side with the teacher. The way I see it, Guwon’s point of view, if not incorrect, is kind of brutal. But if you actually understand that they are in a post-apocalyptic world with zombies and who knows what else out there, you can’t go around saving everyone.. especially when you don’t know who you’re saving. The idea of saving only the “useful,” for example, a doctor or the teacher himself, is actually more practical and reasonable.