It's not actually clear to me that she put national security at risk. During the recent U.S. State Department hack, an unclassified email system was compromised. Most of the emails Clinton sent (99.6%) were not classified at the time, and could have been sent on that email system. Even most of those that were classified were not marked as such, and still could have been sent on the unclassified system in error (which would not have been illegal either, assuming there was no intent).

For me, a better argument has to do with minimizing exposure to freedom of information.