Non extradition due to death penalty might work anywhere in Europe. It might not if US says it won't execute Snowden (if they lie, then no one will ever be extradited on that basis again, as that would be illegal from then on).
For example: http://en.wikipedia.org/wiki/Noshir_Gowadia http://en.wikipedia.org/wiki/Robert_Hanssen http://en.wikipedia.org/wiki/Walter_Kendall_Myers http://en.wikipedia.org/wiki/Harold_James_Nicholson
They'd much rather him rot without trial in Guantanamo for the rest of his life, as to serve a better lesson to anyone thinking of following his footsteps.
See: holding Manning for years without trial, killing American citizens without trail (overseas)
After all the bull*hit about "enemy combatants", my faith in the separation of powers in the US is quite nil.