Only someone who hasn't followed the meat of these NSA leaks could leap to such absurd conclusions.
"What is that devastating piece of information that Snowden says he knows that could be so painful for the American government?"
Snowden did not make any such statement. You can end the ridiculous speculation exercise right there. What this website you've linked to may be referencing are recent misquotations and out of context statement of both Snowden and Greenwald by the media to play up some sort of blackmail scenario whereby Snowden is going to unleash the US Gov's "worst nightmare" via a huge data dump if he is to be assassinated, disappeared, ect.
A recent Reuters article doing just this prompted Glenn Greenwald to directly address their shit article:
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