In the graveyard Voldemort says about his death eaters "One, too cowardly to return...he will pay." I believe he is referring too Snape. "...one, who I believe has left me forever...he will be killed, of course." Here he is referreing to Karkaroff becasue he takes flight the same evening. Also Snape must still be working with Voldemort in order to spy on him for Dumbledore.