and even if both of them weren't lost in a morass of darkness, Buffy was.

You know, I've heard people talk about how in that season Spike was trying to drag Buffy into the darkness to join him, but I always saw it as the opposite. She was the one dragging him down. though I can't exactly say he was some kind of "victim" in it, because I think allowing her to treat him and the relationship they had the way she did was very consciously done. Either as a misguided attempt to help her, or a way to keep her with him in any way possible.