Killing off the solo companion (as compared to a secondary companion like Jack) is incredibly brutal, since she's meant to be the kid's POV character, and it's never been done in the whole 44 years of Doctor Who - the one time it did happen, when Peri got killed off, they had to retcon her back to life two stories later. This makes me vacillate between "Rusty was being honest when he said Martha wouldn't be killed off, he wouldn't go there" and "yes he would, for the shock value". :(