@CHIZU no, trauma doesn't excuse, but it can make his actions more valid and easy to forgive considering everything he went through. We have to see this from his POV. an abusive father who was never home, then a mother who always hopped guys then ended up killing the father and raping Sangwoo, which landed in her death. What she did to him took a heavy toll on his viewpoint and mindset, which is probably why some of the comments he makes are very childish and-or uncalled for. Sangwoo's mentality is broken and more twisted than others, and he holds no fault for what he's done. His actions are driven by the trauma he endured from his family and not for any other reason you may think. It is PURELY because of what has happened to him. Sangwoo has no control over his episodes which we've all seen him have,, like crying out of nowhere, getting mad for no reason, crashing out over something as small as dropping a plate on the floor. What he does and what he says is a reflection of what his mother and father did to him, meaning he is only repeating what he has seen. It is not because he is a vile person, but he was raised around vile people who did nothing but hurt him. he's not just some crazy guy with a bad past he's mentally and physically traumatized to the point he has psychotic episodes he can't control. There has been many chapters where he's suddenly snapped out of whatever trance placed in, realizing the depth of his actions and regretting them internally but being too proud to speak up.
With the "every abused kid should become a serial killer" mockery statement, you're not understanding the fact this was more than just abuse. It was something deeper that really screwed him up. Plus, not to mention: he witnessed his mother kill his father,, then was physically abused by her once she had officially lost her mind.