I think it was his mom. In the flashback there was a mole on left side of her face but in chapter 13 it was on the right. The mole is such a minor detail though that can be overlooked, it could be that Koogi just didn't draw it in the right spot ._.
There hasn't been a whole lot of details given about Sangwoo and his relationship with his mother but I think he had a somewhat loving relationship with her, even if he did kill her. Sangwoo had stated that Bum reminded him of his mother and comforts him at times so it would be too strange for him to think of her in that moment. I think that his mom did something that made him mad resulting in him killing her. Sangwoo might have viewed Bum running away as a similar form of betrayal and wanted to kill him as well.
Ok, I'd really love to know how he covers up his crimes (〜￣△￣)〜
Does anyone else think the girl he was referring to in chapter 13 was his mother?
Cause in flashback with his parents his mother had a mole on her cheek? :s #justaasking