Lots of things could have happened. He could have hunted this kid down, he may have shot him in the back if the kid didn't turn around. Perhaps confronted with shooting the kid face to face he lost his balance? Not everyone can shoot an unarmed person when actually confronted. If that's the case it's premeditated murder. It could have been self defense and it could have been something in between.
Being a "Thug" doesn't give someone the right to hunt them down and shoot them just because they were a "Thug". An unarmed kid was shoot to death by a guy who hunted him with a gun. The news media turned this into something a little crazy. Maybe in a courtroom the jury with the help of the Judge and the attorneys will figure this thing out? Probably not but its worth a shot in the dark.