I thought the same thing, what a cruel thing to do to an innocent child, I hate it when they blame the child for things like that, it's not like he asked to be born ur the one that F%*ked up when u went out with that man and to even have his child, u might not have seen the signs before but as the first red flag was risen that's when u should have left with ur child or do something to get away from him don't blame the child
Maybe it's becos he pulled the scissors out of her that he felt he killed her. Then when you look at it, it seems to him that he had stabbed his mom with the scissors (the trauma might have twisted his memory) and cause him to think like that.
I actually thought Yoseob killed his mother but it seems he believes that because she killed herself specially because of him