Re: My $ 0.02
IF I recall correctly, Voldy's comment contained the word 'seems' - as in '...SEEMS to have' "left me forever". The only reason Snape was not killed was that he could convince Voldemort that despite how it seemed, he had not left him.
Voldemort's actual word is "believe":
‘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 re-entered my service.’
Even given later events (e.g. what we see in Prince's Tale) I think it remains ambiguous which one is "too cowardly" and which he believes has "left [him] forever". I'm inclined to think that Karkaroff is the former and Severus the latter (in which case, yes, he would have had to convince Voldemort out of that "of course" death sentence), but within the text itself it could go either way.
Voldemort's actual word is "believe":
‘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 re-entered my service.’
Even given later events (e.g. what we see in Prince's Tale) I think it remains ambiguous which one is "too cowardly" and which he believes has "left [him] forever". I'm inclined to think that Karkaroff is the former and Severus the latter (in which case, yes, he would have had to convince Voldemort out of that "of course" death sentence), but within the text itself it could go either way.