I took psychology in high school, And I think Sangwoo is two things. First he is a psychopath. Which answers the question if he loved Bum. Psychopaths don't have the capacity to love. Second he is a sadist, which is a person having a sexual desire to hurt someone physically. Now what does Bum have? I think Bum has Borderline Personality Disorder. This is when a person has a mental disorder characterized by unstable moods, behavior, and relationships. I think he might also have some PTSD (Post traumatic stress disorder). PTSD is anxiety and flashbacks triggered by a traumatic event. Which for Bum the event would have been the times he was sexually assaulted by his uncle.
Hopefully this helped.
When I tell you my heart was pounding so much, like my heart hurts right now, and I love it. (≧∀≦)