They were never good for each other. Sangwoo abused Bum and did all kind fucked up shit to him while Bum looks like Sangwoo's mom which triggers him more and more and the realization that he's not his mom seems to have changed him, making him even more unexpectable.
Actually is the other way around.
They both are too damaged to be with someone who is.. normal.
But the fact is that since Bum came to Sangwoo He stopped serial killing and He is now less aggresive.
Bum on the other hand, has became more assertive and start asking things for himself.
This is NOT ideal. This is FICTION, it's REALLY hard that something like this works in reality... but could be.
Of course Sangwoo is now seeming destroyed, but that's because finally He is facing his thraumas instead of just smashing them on corpses. It's kind of what therapy would do.
Again: This is NOT ideal and both have suffered a lot by this, but now is the time when they are working out better for each other since they meet.
Yeah, I started to feel pity for Sangwoo recently with how.... Well, vulnerable he's gotten. It's exactly what you said: he's finally facing his traumas instead of escaping his thoughts and memories. And man, watching him struggle from the sidelines kind of make you wonder whether this progress is for the better or for the worse, y'know?
On the other hand, it is pretty cool to see Bum taking some semblance of confidence and authority. But I kinda see glimpses of sadomasochism from him and I'm half amused and half worried. It's like when someone seems potentially dangerous and somehow this ambiguity excites you and scares you at the same time.
Is Bum really good for Sangwoo? Is Sangwoo really good for Bum? The deeper we get into the story the more I think they're both dragging each other down into the deep hole together.