Apparently he arrested a thief, but was surprised by several others. You cannot see the actual murder, but it seems he is killed by his own gun. I do not think any US officer would be so close to the perp.
Damn you can see the guy he was holding at gunpoint rack the slide on the officers gun and put it right to his head. I assume he should have had that guy on his knees away from him with his back to a wall watching the entrance while he radios for backup.