Are you aware that after Trayvon ran off into the darkness that Mr. Zimmerman ended his call to 911 and headed back to his truck? Only problem was Mr. Zimmerman didn’t make it back to his truck because Trayvon came back and attacked him.
If someone ambushed you in the dark, was on top of you punching you in the face, you’re screaming for help and no one will come out to help because they are all too scared to do so, what are you going to do? Sit there and just let it happen and possibly die in the process of the beating? Or are you going to defend yourself with the knife or gun you might be carrying?
We don’t know if Zimmerman was headed back to his truck or continued pursuit after his 911 call. You only have Zimmerman’s word on that point.