If it can be proved he turned back to as was going to the truck then it would be he who was stalked and attacked but that is the real question isn’t it and maybe the most critical part of this case.
the angle and proximity of the gunshot would make the case for whether Zimmerman was in a struggle or not
If his story is that teenager was on top of him pummeling him when they began to struggle for the gun, then the gunshot wound and other trace evidence would back up his story.
Or not.