Reading some articles from when he was originally convicted, it looks like the thing that sunk him was eyewitness testimony from one of his fellow Guard members who said SGT Miller told the Afghani that he’d shoot the guy if he lied to him. Then at some point he was straddling the guy on the ground and fired at point blank range in some kind of execution move.
So was the eyewitness lying? Was he mistaken? Is he going to be punished? This whole thing is weird.
Lying. Post 37 explains it very succinctly.