Exactly.
There's no difference between an excuse and an explanation when someone acts the way they were raised, if you're raised in a society of cannibals you have no moral standard that rules out cannibalism, if you're raised in a psychotic, abusive, murderous household, you have no standard for any normalcy outside of that. Is it illegal and immoral by the majority of modern society's standards? Yep, but that's just the way he was raised. Abusive upbringings are present in almost every serial killer and domestic abuser's life, when you're brought up on violence during your pivotal developmental stages, that violence often continues throughout your life with others.
it's not that Sangwoo doesn't know how to control his impulses. He knows killing is wrong, that's why he created his outward persona of the good boy and it works perfectly in society because no one beside Seungbae suspects of him. It's true that most serial killers had an abusive childhood, but not all the victims of abuse become abusers themselves. And not all the abusers were abused too.
He controls them opportunistically and manipulatively, just like his mother. She'd put on the "poor sweetheart" act to make him feel bad for her attempted murder of himself and then turn around and murder/demonize the parent who was trying to protect him from her. In a sense, Sangwoo thinks that his violent tendencies come from his father, but he's much more like his mother than he could possibly realize because she was the really fucked up one from the jump. It's some twisted shit.
Yes but this doesn't happen in every situation. Bum was abused and raped all his life and he hasn't become an abuser and rapist or a murderer ( until Sangwoo forced him in the situation). He didn't want his rapist uncle dead, because it's not in his nature to hurt people. Sangwoo's past is not an excuse, he couldn't be excused because he has responsability of his actions as an adult
But he was more capable of it than a well-adjusted person with an average upbringing would be. He also wasn't directly exposed to murder from a young age as Sangwoo was. His abuse was mainly sexually charged and manifested in an unhealthy sexual lifestyle, like stalking the person you like and breaking into their house to try to get closer to them. Every action has a purpose, regardless of how pointless they might seem at a place. That's a big theme of the story that the author is trying to convey.
I think some of you are forgetting that past is an explanation, not an excuse.