My $ 0.02
(Anonymous)
I'm not sure one way or the other, but: Voldemort says he's going to kill the one who's left him forever. Snape returned quite soon after that announcement. How would he have not only persuaded Voldemort to spare his life, but convinced him of his loyalty all over again? Karkaroff made it clear that he was done with Death Eating, and he was killed eventually, so he could be the one who left. Also, as was pointed out recently, for some reason it's a recurring pattern in the books that someone calls Snape a coward at the moment he least deserves it.
-L
-L