There are two main moments in this case that are extremely important.
. . . and really only two despite all the hoopla.
Either man could have started the fight, but this is ACTUALLY, not an element that is important to the legitimacy of the self defense claim.
A fist fight can occur where both parties claim the other started it. If at any time one person is able to disable, knock down, or otherwise prevent the other party from effectively continuing in the fight then the "fair" part of the fight is over. It is now an unequal force equation.
Once this occurs the person who was able to render their opponent disabled, or knock them out of an equal force stand up fight, then they have the ability to disengage.
This is a pivotal moment in any physical altercation. The right thing to do is disengage. The wrong thing to do is to continue to attack the person who has been knocked down, or rendered disabled.
In this particular case, the most pivotal piece of evidence is the witness who says he saw Zimmerman on his back, and Martin continuing the attack.
When this person came out and claims to have yelled for Martin to stop his attack, this was ANOTHER point at which Martin had a choice to end the encounter. With a third party on the scene it makes sense to stop attacking and seek an establishment of the facts.
So twice he had an option to end the attack, regardless of how the scenario started this means that Martin through his own choices and actions setup a scenario where the person being attacked did not know that his attacker would stop. This establishes an objective reasonable belief that deadly force was needed to stop the attack. People die from beatings all the time, if the beating is not going to stop, you are going to die. < objective.
This is so over the top damning that any third party who understands the law would be hard pressed to even imagine any possible evidence could over turn such a thing.
These are the facts which make an arrest on Zimmerman highly unlikely to ever occur for the crime of manslaughter.
Is Zim a busy body jerk who ignored the basic tenets we are all taught about concealed carry, and neighborhood watch, sure that might be true, but it does not remove his ability to defend against a continuous assault by a party who will not stop the attack.