
and bum would still be on sangwoo's side, despite being trashed and all.

That was my first line of thinking too but I don't think that's the case. For me it kinda felt that Sangwoo is reenacting the scenarios that happened when he buried his dad with Bum as his mom (or maybe himself? I'm not yet sure on that). He probably did everything (have Bum kill the girl, the mountain trip) as a kind of 'ceremony' of Bum being integrated into his life. That's how I think it is but I could be wrong

I hope not, that would be too boring.

I agree, perhaps Sangwoo is seeking companionship. He's been alone for so long, no man is an island. And what better way to get a friend than murdering people together lol. Kidding aside though, I agree with you. The murder of Jieun was Sangwoo's way of integrating Bum into his life. He even got upset when Bum said that he didn't feel anything during the kill. Sangwoo wanted him to enjoy it, to make him have a feeling of relief from it like Sangwoo does. However, I think Sangwoo should have used a different approach, like, idk... killing Bum's uncle instead of the girl?

bum was a stalker initially, and he seems to be obsessed with sangwoo. in first chapter, he finds sangwoo to be kind for kissing him with his mouthful of porridge while having a corpse lying beside him. sangwoo can feed bum with affection, manipulate him and still believe sangwoo isn't a bad person.
ima call it now bum will be charged with the murder. Sangwoo is just setting him up lovely AF