
What if Guwon - who is not understanding human emotions - saw one day someone killing another person in similar situation, and just thinks that is the way of doing ? He is just like a kid copying behaviour he saw. And Since no one corrected him... he is just doing that couse he doesn't know the proper way.
He clearly didn't enjoy killing dad... And he was sad and shocked when Teach told him it was wrong.
I still believe he is a good guy... Just... he needs to learn.

I feel it's not the whole story. We just heard one side of the story.
It feels like Guwon doesn't know emotions, what is right and wrong. That's why he was so suprised when Teach slaped him.
No one told him it was wrong before.
And the towns people didn't correct his mindset becouse of fear.
Teach is the first and only one telling him that and teaching him human emotions.
And about the end of the world if Guwon dies... he is not human but some sort of entity. it' s 100% tied to his lack of understanding emotions.
So it's "ohh in a twink gone" when you Fall in love ??!?!! I Wonder what Byul's reaction is going to be when he will start getting buff