I'm glad someone pointed this out. That scene- him freaking out over Bum bleeding, getting a flashback of his mom and then desperately wishing to not turn out like his father- shines a whole new light on Sangwoo's character. It's a complete contrast to what we've seen of him thus far. It adds a depth to him. A depth many would be reluctant to accept considering his already established image as a merciless murderer. Also it's the first glimpse we get to Sangwoo to have had been a victim himself. Despite this revelation being far from nullifying our impressions of his psychopathic killings (which up until now, have only seemed like a blatant lust for blood), it's a detail we can't ignore either. It definitely creates conflicting feelings towards the character and makes it harder to make up our minds about him. It has us more intrigued and has us wondering what could've happened to him that made him who he is now.
I get this is to late to say but i really dont think sangwoo is all bad i mean after the rough childhood he had he obviously developed some psychological issues that only increased after the death of his parents cause going back and rereading chapter 27 he seems indifferent when bum collapsed and was bleeding out but after walking away and coming back he seemed completely shocked and scared which i believe is the real sangwoo and the sangwoo which is cruel and pscho is really how he interpreted his father to be and he either took his fathers personality to somehow protect himself or just wasnt really able to fully developed his own personality