yeah i could totally see that happening and he also mention that Bum reminded him of his mother
his mother abandoning him could also explain his trust issues with women
well in previous chapters with the little brown table we got a glimpse of little "sangwoo" and his mother being abused by his father. And in i think it was chapter 2 the officer also mentioned that it was an inside killing but with no motive of anyone wanting to abandon the other. I personally think sangwoo loved his mother a lot but felt as if she was making herself helpless to his abusive father. so that made him hate her for not being able to stand for herself. see sangwoo likes it when you defy him, it gives him a thrill but when you don't he feels as if you're useless. thats my word. but im not the writer so ill wait for the whole scenario to unfold.
I personally agree with almost everything you said, I cant wait to find out the truth.
During reading chapter 13 when Sangwoo was looking for Yoon Bum and he was out on the streets, he remembered when he killed his mother( http://www.mangago.me/read-manga/killing_stalking/mf/c013/45/). At the time he was thinking about doing the same thing to Yoon Bum because he ran away (abandoned him). I might be wrong but this moment suggested to me that his parents might have been getting a divorce or Sangwoo mother was going to move away without him (abandon him). Since he thought about her while he was looking for him and wanted to kill him for running away (after all the things he did) he might have been comparing them together (as if there was something similar about them).The idea of him killing his mother because she wanted to abandon him makes sense to me but it only popped into my mind recently so I might be missing something.
Tell me your opinions about my little theory so we can come up to a great conclusion.