Quote from 377OHMS:
As I said, I know I'm wasting my time but I have a few minutes free.
Zimmerman's account is that Martin just showed up and said "Do you have a fucking problem?" and that he (GZ) said "no" and reached for his cellphone and was slammed in the face. Zimmeran said that Martin jumped on top of him and began slamming his head into the sidewalk. Zimmerman said that he tried to move his head onto the grass so that he would be killed but that move exposed his gun. When Martin saw the gun he said "You are going to die now" or "You are going to die today" and Zimmerman decided to shoot Martin which he did from the ground. That suggests Zimmerman had a round chambered and the safety on which is not the way I would carry a gun but there it is.
Of course, Zimmerman's dad, brother and friend are all just relating Zimmermans version of events. The adult eyewitness did see Martin on top of Zimmerman and smashing his head onto the concrete.
I don't expect you to believe the eyewitness, the police or the paramedics but don't expect there to be an arrest as there simply isn't any cause that the police can use to arrest Zimmerman.