It was night. Zimmerman was a stranger to Martin. Zimmerman followed Martin. At no point did Zimmerman identify himself nor announce his intentions. Zimmerman made a quick reach into his pocket.
We have two divergent theories to the physical start of the altercation: George's version above and DeeDee's version with Trayvon to saying "get off, get off". I think we can discard Georges first version that he was struck from behind since that is incompatible with his later statements. Both versions lead to the same likely response: Trayvon hits George in the face and George falls to the ground.
There is the possibility that both accounts are right though George is leaving out the critical detail of how he gets so far past the Tee. George meets Trayvon, words are exchanged, George reaches into his pocket, Trayvon starts to take off, George detains Trayvon then Trayvon says "Get off" and hits George in the face.
All accounts lead to George on the ground where Trayvon straddles him.
At this point, George is claiming that Trayvon is beating him and pounding his head into the sidewalk. The physical evidence: lack of defensive wounds on George, lack of blood and scalp on the sidewalk, lack of blood and injuries on Trayvon's hands, etc. simply doesn't support this. Alternatively, Trayvon is controlling George by pinning his arms outstretched as one witness reported. This is still compatible with what George claims he felt as after being struck in the face and falling backwards hitting his head on the ground or sidewalk, George's head would be pounding internally and with each pound he would think his head was being smashed into the hard sidewalk.
Who was screaming for help? This may not be possible to determine. Trayvon could have taken off as soon as George went down. With Trayvon physically restraining George and not beating the crap out of him, Trayvon would have a desire for help to arrive to stop the fight and prevent retaliation. George is defenseless in this position and also desires help. I would expect George to experience serious head pain each time he forced a scream but George never mentions this.
In the end, the neighbor steps out, witnesses the fight, says he is calling the police and steps back inside. In the position that they were lying on the grass, George would be able to see this. But Trayvon's back would be towards the neighbor. Trayvon could have heard the neighbor say he was calling the police but would not have seen him retreat back into his home.
It is at this time that George is thinking about his gun. He says he thinks that Trayvon could have seen it. George hears the words "You're going to die tonight". We are supposed to think that Trayvon said those words but really?! Trayvon doesn't have the gun, Trayvon cannot see the gun, Trayvon thinks that there is a witness standing behind him. I think those words were in George's head.
George knows he has a gun and the only way for George to access that gun is for Trayvon to be backing off. Only then can George pull the gun, plant it in Trayvon's chest and pull the trigger while there are no witnesses.