It is, of course, that highly predictable paradox that Charles is avoiding tangling with -- keeping it abstract instead of giving Severus something tangible to argue. If he doesn't put himself out there, he doesn't have to be told that he's unreliable, without ethic, that past efforts to stand up for Severus didn't happen, that he's too much of someone's patient to function also as their friend.

That it wouldn't be good enough. Charles looks back, just as flatly, before he goes to leave, hands folded neutral behind him.