It seems that one is defining the term 'hero' in a rather odd way, if one can't consider a deserter a hero. Let's look first at what desertion from the U.S. military means, in terms of actions and possible consequences, and then more specifically at Mr. Bergdahl's particular situation, or at least what is currently known of it.
When a soldier decides to desert, he or she individually opposes the strongest government in the world, and that government's military, which purports to all but own the soldier. Often, the reason for the desertion is military life itself, complete disillusionment with the war the soldier has been forced to fight, or a combination of both.
Military life has long been a cause of desertion. Two examples from different time periods will illustrate this, although these are only examples; the government's actions that motivated these behaviors have permeated the U.S. military from the American Revolution to the present time.