As a practical matter, he can't come home. Not even the president can promise his immunity from prosecution -- one day there will be another president. The DOJ could drop all charges. And then impose new ones once he returns. And that is to say nothing of the possibility of his being killed, with blame cast on some nut job, and with all sorts of manufactured dirt on him released afterward.
Put yourself in Snowden's shoes. What could anyone in the government say to you that would convince you it was safe to return to the USA?
> Not even the president can promise his immunity from prosecution -- one day there will be another president.
The President has the Constitutional power to issue pardons, it does not grant the President (whether the same President or any subsequent one, or anyone else for that matter) power to retract them.
> The DOJ could drop all charges. And then impose new ones once he returns.
Not to the extent that the new ones are covered within the scope of a Presidential pardon.
> And that is to say nothing of the possibility of his being killed, with blame cast on some nut job, and with all sorts of manufactured dirt on him released afterward.
To the extent that could happen (at the hands of the government or otherwise) if he returned, it could happen if he didn't. Its not exactly like no one is ever killed -- including by the US Government -- outside of the United States.
It's easier to retaliate when the target lives just next to you than in another country. But (unless Snowden starts messing with the government again) more likely than intentional government-sponsored execution (er, "accident") is that some nutjob (er, "patriot") will take matters into their own hands. And that is vastly more likely when you're in the country and not abroad.
> To the extent that could happen (at the hands of the government or otherwise) if he returned, it could happen if he didn't. Its not exactly like no one is ever killed -- including by the US Government -- outside of the United States.
If it happens outside the United States, there's plausible deniability. If it happens in the US, there'll be far more scrutiny.
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).
Put yourself in Snowden's shoes. What could anyone in the government say to you that would convince you it was safe to return to the USA?