He walked away from his guard post, while his countrymen were sleeping; he exposed them to possible lethal harm by his actions. He sought out the enemy, to join with them, against his own countrymen.
Pvt Berdahl laid down his Squad Automatic Weapon (SAW) a light machine gun that provides significant firepower for the defense of his platoon's position. The absence of which leaves an unprotected hole in the perimeter defense at night.
No eyes and no weapon where his comrades relied on Bergdahl to watch their flanks and back.
After laying down the rest of his kit he had to sneak from his firing position at night and then navigate the concertina wire enclosing the position, all without alerting his comrades.
All his actions were carefully considered and planned.
Pvt Bergdahl should have his court martial and then hang from the gallows.