Presidential pardons stick because of double jeopardy. If Snowden is pardoned for the crimes for which he has been indicted, he cannot be indicted for them again.
If Obama pardoned Snowden on his last day in office (as I hope he does), Snowden could come home.
If murder is the concern, Snowden will not be safe anywhere. Do you think that the CIA doesn't have people in Moscow?
I don't think double jeopardy enters into it. You can't prosecute someone for something they've been pardoned of, even if they were never indicted before (and pardons without indictment have happened plenty of times).
If Obama pardoned Snowden on his last day in office (as I hope he does), Snowden could come home.
If murder is the concern, Snowden will not be safe anywhere. Do you think that the CIA doesn't have people in Moscow?