I was in the army and it did not matter where you were, guards were posted for the protection of everyone. Anyone that would try to leave was stopped. I don’t think the Taliban would send some of their men just to kidnap a lone soldier. He wan't even an important guy. Te only info he could have given them is how many soldiers were in the camp and what kind of weapons they have. No, I would say there was something fishy going on. I really believe that sergeant Bergdhal left of his own accord for some reason or other.
Another thing he was over there not to kiss the Taliban but to kill them and they knew this. They also knew or should have realized that even though he had given up his faith to become a servant of the Islamic faith he could have agreed to become a follower just to save himself. In any case this man was with them according to the reports for 5 years? And he's still alive and well?
Now I am not saying that he definitely left of his own accord but it seems very likely. And that would make him a traitor. My opinions are based on what my own experience in the armed forces.