Who is the 'one that never will return' Death Eater in Harry Potter?

Answer 1

In Chapter 33 of Harry Potter and the Goblet of Fire("The Death Eaters") Voldemort says, "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..." (USA Scholastic hardcover edition, p651).

The "one, too cowardly to return" is Karkaroff. The other, "who I believe has left me forever" is Snape.

When Voldemort first got his body back in the graveyard, he summoned all his Death Eaters. Snape didn't come when Voldemort called. Snape came to Voldemort two hours later, on Dumbledore's orders. Snape bluffed Voldemort, basically. As he himself says to Bellatrix and Narcissa in the "Spinner's End" chapter of Harry Potter and the Half-Blood Prince, "I returned two hours later. I returned on Dumbledore's orders...By waiting two hours, just two hours, I ensured that I could remain at Hogwarts as a spy! By allowing Dumbledore to think that I was only returning to the Dark Lord's side because I was ordered to, I have been able to pass information on Dumbledore and the Order of the Phoenix ever since!" (USA Scholastic hardcover edition, p28).

So, Snape lied, to Voldemort, and to Bellatrix and Narcissa. He pretended that he was spying on Dumbledore for Voldemort, when in reality he was doing the reverse: spying on Voldemort for Dumbledore.