I think he may have picked up the stun gun that had been fired. He should have never done that, obviously. Placing it next to the body makes it appear that the victim had possession of the stun gun at the time of the shooting.
The arrested officer probably told his former attorney what had happened and then when the video came out which contradicted said version of events - bye-bye attorney.
Hard to say. He’s toast, though - should be, IMO.