“And here we have six missing Death Eaters . . . three dead in my service. One, too cowardly to return . . . he will pay. One, who I believe has left me forever . . . he will be killed, of course . . . and one, who remains my most faithful servant, and who has already reentered my service.”
I used to wonder if the "faithful servant" was Snape or Crouch, this following paragraphs make it clear Snape is the one who "has left forever".
