Maybe it's just about Snowden revealing more names, more details about how, that will cause uproars in more countries like what happened in Brazil and Germany. But maybe it's something more.
Is it fear of someone leaking proof of the big lies they've been telling, about bin Laden and the War on Terror? Details of their plans for taking full power over the world, or for a managed collapse of the world economy? Details of their plans for starting and winning World War III? Or of horrible new weapons systems they've been developing in secret, to use against ... who?
That would b a threat to be deterred at any cost.