Security guru Bruce Schneier suggests Snowden might not have considered all the likely outcomes:
Edward Snowden has set up a dead man's switch. He's distributed encrypted copies of his document trove to various people, and has set up some sort of automatic system to distribute the key, should something happen to him.
Dead man's switches have a long history, both for safety (the machinery automatically stops if the operator's hand goes slack) and security reasons. WikiLeaks did the same thing with the State Department cables.
I'm not sure he's thought this through, though. I would be more worried that someone would kill me in order to get the documents released than I would be that someone would kill me to prevent the documents from being released. Any real-world situation involves multiple adversaries, and it's important to keep all of them in mind when designing a security system.
Possibly spending a few years at the Moscow airport might be his safest option. But then again, his whole strategy seemed flawed from the start.