The only way it would be even remotely safe for Snowden to return would be if he received a presidential pardon for any "crimes" he committed while revealing the NSA spying activities. However, do President will do this for fear of being branded a "in league with a traitor." Even if a President were to pardon Snowden, you can bet he'd be under constant surveillance and the first time he stepped out of line (say, drove 5mph over the speed limit), he'd wind up getting the maximum penalty. This could be used to harass him and make his life miserable. He could also wind up the victim of a "horrible freak accident." He's angered a lot of people who have a lot of power and his actions have made it slightly harder for those people to get even more power. This isn't the kind of thing that just vanishes because one person says he's pardoned.