Hmmm

I believe he wanted bum to kill his past relationship that still affected him.
Sangwoo doesnt like bums uncle based from what he heard about him from bum.(possibly even a little jealous) And wanted to help bum kill his uncle. Not understanding that bum is too scared to do such a thing. His a scaredy cat. Sangwoo got mad because bum didnt like his gift. And left him alone. Most likely to sulk. And then he realised how bum must have felt and came back to clean up the mess. Its the thought that counts. These 2 r so cute

Hmm...... Well said (　＾∇＾)