He evidently left his vehicle with a loaded gun. Whether there was any interaction before he murdered the youth,is unknown,but i seriously doubt a kid attacks a much larger man who's got a gun and is acting crazy.
That the racist cops covered up for a guy who was a wannabe cop...or a bully/vigilante...seems probable.
Phone and 911 records make it clear Zimmerman was tailing Martin. There's no question who initiated things or who is the killer. In this...the "Castle" law does not even kick in, though...I guess in Sanford if you see a black kid wearing a hoodie....you just blaze away.
I wonder if they consider lynchings to be legal there? That's about what this was.