Trumps comments on this case have been bad, but perhaps even worse was the outright meddling in the case perpetrated by Senator John McCain.  Back when "administrative punishment" (Article 15) was being contemplated for Bergdahl, McCain came out and threatened that the Senate Armed Services committee (which McCain chairs) would hold hearings on the issue if Berghdahl was not punished sufficiently.  A few weeks later General Robert Abrams sent Bergdahl's case to General Court Martial, rather than article 15. 
 Since McCains Armed Services committee is also required to approve promotions and assignments of senior officers, his threat was seen as exerting undue influence on the judicial case.  Of course everyone from the Judge to General Abrams denies that McCain's (or Trump's) threats influence them, the fact remains that if either Trump or McCain feel that Bergdahl get off too lightly, it could well mean the end of the lenient officers careers.  McCain at least should have known better.