I don't really trust her and I don't think her presence is welcomed in Sangwoo's life since he's... Idk, acting possessive somehow? Like he's scared of losing Bum and with all the shit his mind is going through, he's mentally and physically exhausted and I think he's become even more paranoid than before. And because of this that Sangwoo might be driven to either harm the girl, or harm Bum. However, you're right, she doesn't really need or deserve to die "just because" or "out of whim" by neither Sangwoo nor Bum. I just don't think it's the right move, strategically, right at this exact moment.
The only explanation i can think of rn is that I'm ded and I'm in hell or another realm because i swer i did not just see Real° Sangwoo Tears . Also is it weird i dont want that Woman to die ? I mean she might,,,but she's stirring some friendship(?) with my Yoonbum and i think he needs some rn. *flippin tables* FUCK I DONT KNOW