Do you mean ch.22? I think that was just a nightmare he had. And again, his memory is weird and could be distorted so there's still a lot of possibilities! Thats why the story is so good imo
The dad is probably dead, but there's also the small possibility that he's alive if my theory of teach's distorted memories are right :3
If the dad's still alive, I think it could be used as something to mend the relationship between Guwon and teach since there is almost next to nothing holding them together. Teach will find out Guwon didn't lie to him and maybe trust him more or idk something like that (though i think it's unlikely the story will go this route)

Nor does it matter because I think Guwon has done worse
Like why does it matter if he killed teach's dad specifically, he killed a lot of others already. I don't think that should be a point of contention
And from what chapter 0 showed us, that dead guy in teach's house likely wasn't his father, it was some dude trying to rob the place.
My theory is teach might've killed his own father, considering he had a weird thing with his memory and trying to strangle people in his sleep. But I am interested to read what others think because the plot is pretty good and it's still too early in the story to be sure of anything