I don't think it's as abstract as fate. What kills Cassie is a genetic flaw - her physical make-up. Could it have been avoided? Possibly - had someone made the connection between mind and body. Re-establishing these connections is, IMO, S7's priority. Had Cassie's parents not - with the best of intentions - kept her history from her perhaps she would have made the connections for herself? If so, would she have sought help in time? Is the text making a parallel between Cassie and Buffy? Did Giles (or more likely the council) keep the demon origins of the Slayer from her in order to have her on the side of "light"? In fact, isn't it true that Buffy really "belongs in the shadows" as Spike insisted she did? In fact - and this has just occurred to me - perhaps Buffy was "born wrong" the first time she was born but the nurture she received from Joyce (and Hank) made her more "human" then she was. Joyce wasn't there in her third "re-birth" to ease her through it. This would tie in nicely with the questions raised in Tabula Rasa.fate still remains more powerful than her, than everyone. Fate is the one that rules in the end, taking out Cassie even though Buffy and the others strived to save her till the very end.
Thanks for this - it's given me a new perspective on the episode.