That has probably been answered by the description given by Gellman of his initial corespondance with Snowden, and the story given by Greenwald, MacAskill, and Gellman about the HK meeting with Snowden. (see: this guardian story and this PBS documentary)
As Barton Gellman described:
He believed he was risking his freedom, and possibly his life. And he warned me, as well, that if the U.S. intelligence community believed that by getting rid of me they could prevent the story from happening, he said that my life would be at risk.
Pubishing may be what saved their lives, because it makes their death a lot less profitable, and far more visible. The risk today is probably not zero, but it’s a lot lower than it was a year ago.
