I had to post from three sources to get enough FACTS to show why those assuming guilt are premature.
Your comment has merit. I would like to add that in fact, a 911 operator or police dispatcher has no authority to order anyone to back off. Only a police officer in person, on the scene, has that authority. Mr. Zimmerman was not in violation of the law when he followed Mr. Martin.
In my opinion it was unwise of him to get close.
The synthesis I see from multiple sources is that Zimmerman did back off. He turned his back on Martin to return to his truck. Martin jumped him from behind. An eye witness observed a man in a red sweater (Zimmerman) on his back with another man (Martin) beating him. Zimmerman called for help. The eye witness told Martin to stop, then headed inside his home to call 911. One the way upstairs, the eye witness heard a gun shot. He looked out the window and saw the man who was doing the beating on the grass, presumably dead.
Martin is no "child". He made the decision to attack Zimmerman when he was leaving the scene.