I think he wants Bum's seed in the dead body so when the police find the body later, Bum will be implicated. At the same time, he is brainwashing Bum to think everything is Bum's fault. The plan is to make Bum the fall guy for everything. If the police get close, then Bum will be thrown to the wolves. But some loose ends that were not planned may unravel like that car video of Bum getting beat up on the windshield..
I hadn't thought of him trying to blame Bum, but it's a pretty good theory.
Like an idiot I keep hoping he'll actually care for Bum, but then when he does the next horrific thing like put a dead girl in bed with Bum, I remember he's psychotic and incapable of caring for anyone but himself.
I'm also betting blaming Bum would be tough because he's scrawny and beat up.
Telling people Bum kidnapped and killed a bunch of people would be hard to sell to the police just based on his lack of physical strength.
I also wonder about the cell phones. If Sangwoo's neighborhood is really deserted, unless he disables his victims' before he takes them base to his house, I imagine tracking the cell phones would lead right to him.
I'm pretty sure I'm scarred for life.