Anything I say is speculative, of course, but perhaps Ms Heche's own struggles with mental illness made Catherine the perfect role for her and the timing was such that she was able to channel her own personality traits into a performance that was clear and compelling.
I enjoyed her very much in the films I saw her in and found her a very penetrating and deep actress. I would rather not have been privy to the private information that came out about her (beginning with her relationship with Ellen DeGeneres) and just enjoyed her work.
It is a terribly sad ending to a life tinged with torment and pain. And saying that in no way diminishes the effects her crash had on the home and its resident.