Have you listened to the 911 call from Zimmerman. He stayed in his truck until TM came at him and then took off running. Zimmerman did get out to try and see where TM went. Zimmerman then, according to eyewitnesses, stopped the persuit and was walking back to his truck to wait for police. Zimmerman at no time was the aggressorup to this point.
TM then jumped Zimmerman, knocked him to the ground, beat him and that is when Zimmerman, rightly or wrongly, pulled his weapon and shot TM.
Zimmerman did not start the fight and just following someone is not cause to get attacked.