In any case, it's already been established that he broke the law in order to expose a much greater law breaking, but many if not most people believe it to be OK because we also believe that it was impossible to expose the information in any other way. Considering all the miss-steps most whistleblowers made in their activities, Snowden was remarkably careful and clean. He made some calculations on what he needed to collect to successfully expose the crimes he witnessed, and the accuracy of those calculations is up for debate. I could easily come up with several reasons why he might have thought that he needed all those documents while under duress. For example, there may have been evidence of other crimes, and there would be no way he could sift through the documents while still working. Also, he recognized that he needed to leak slowly in order to keep the story afloat, or else he would get buried under propaganda and forgotten, as has happened to other whistleblowers that released all at once.