I think someone has mentioned a few times that the writers planned to have Chris eventually kill Sabrina, but don't take my word for it!
I wouldn't consider Sabrina's death much of a tragedy.
It would certainly be in keeping with the tragedy which follows most of the DS characters.
I just *can't* accept those "subsequent revelations" explained in the spoilers above. That whole mess was done so badly and so confusedly.
As I recall Amy did seem to exert some control over Chris's werewolf character. It seems that Chris would never hurt her. Amy with her psychic abilities would know if she saw the star on Sabrina's forehead would warn her.