He was determined to destroy her, or make her cry, or quite possibly some fucked up combination of both. It was the only reason or conclusion she could come up with for why now, today of all days, the things that had come from him had. If he wasn't threatening to leave her in one way he was threatening to do it in another and it terrified her. Completely and utterly and beyond any sort of thing she had ever felt before. Did he honestly have no idea that she care for him? It might have been twisted and dark and so utterly malformed that even she had a hard time of seeing it at times but surely, after all of this time he had to know. And he had to know how difficult it was for her to admit to such things. She was not an overly emotional person. Not in that way. Nor was she the kind to make profound and verbose exclamations of love, devotion or caring. Merlin knew she wished that once, just once, she could say the things to him that he wanted to hear that he seemed to so desperate to hear but it wasn't who she was. It wasn't in her to do such a things and if she tried it would only come out forced and false sounding and it would accomplish nothing save to make things even worse.
At his words she turned away from him, her back now to him as she stared at the row of windows across the room for several moments before squeezing her eyes shut as tightly as she could, trying to refrain from crying for as long as she possibly could. If he was truly as determined as he seemed to be to leave her then she was determined to keep it together long enough for him to leave. Once he was gone then she could fall to pieces and curse herself for being so fucking stupid where he was concerned but as long as he remained nearby she would not appear in any other way than how she did now.
"If you want to go, go." Her words were soft, empty, hollow and devoid of anything save for a resigned dread that he was going to force her to exist the rest of eternity without him. And if he did that, he might as well just kill her for that is precisely how she would feel without him. Lost and dead.