At the point when Zimmerman left his truck and walked fast, then ran, after Martin (to see where he was going), Martin had a reasonable case for turning and standing his ground.
Martin was attempting to evade harm by walking fast, then running. The perceived threat to his safety, Zimmerman, would not desist. In the immediate aftermath photos taken of Zimmerman, it is seen that Zimmerman's gun holster is visible when he tilts slightly to his left, his short jacket swinging out of the way (the holster on his right side leaves the weapon visible and identifiable just above his waist band). Although the defense keeps harping on how dark it was, there are streetlights through out that condominium complex. Zimmerman was walking quickly, then running, close behind Martin. His jacket would be opening and flowing backwards with air resistance if it was unzipped, or riding up his waist if it was zipped. When you are running with adrenaline pumping from a hostile stranger, it is instinctive to glance back now and then. It is possible Martin saw that gun holster fairly early on.
Upon confrontation, in context of Zimmerman following Martin the entire time, after disobeying the police dispatcher to not follow, it really should not matter as to who threw the first punch. At some point, Martin would have become aware of that pistol. He may have actually felt it brush against his body during the fight. He is then in a serious fight very likely for his life, and he knows it. He probably was angry for the entire duration of Zimmerman following him. It's happened to me, and I'm a pretty mild mannered older female. You feel outraged and scared at the same time, when some unknown man follows you, clearly looking at you, casing you, even saying things to you. It is intimidating and nerve wrecking and infuriating, all at the same time. All Zimmerman needed to do was identify himself as being on neighborhood watch, and ask Martin what address he lived at or who he was going to see there. Even a pissed off, scared, or snotty teen would answer, "Going to my Dad's place!", probably followed by a few choice epithets. I know I would have, at age 17.
Zimmerman did not state who he was or what he was doing to Martin, though, because he had profiled Martin as a criminal immediately. That is established by the 911 recording. Zimmerman followed Martin convinced he would be stopping a criminal. Confronting a criminal. Not waiting for the police. Perhaps he even envisioned, in his established wanna be cop mind, that he would hold the suspect at gunpoint until back up arrived. Zimmerman had no fear of doing that, because he was armed. In Zimmerman's mind, Martin was guilty on sight, and not worth the least bit of benefit of the doubt, or even of proper police procedure in identifying himself, or of neighborhood watch procedures - which is to stay far from any suspicious person and wait for police to arrive on scene. A plain clothes officer would have ID'ed himself as law enforcement at that point; Zimmerman applied twice to police academies, and can be assumed to have some knowledge of what procedure would be. Most civilians are aware that cops have to ID themselves if in plain clothes while commanding a suspect to halt.
Rachel Jeantel's testimony, though she was by turns jaded, snide, and bored and sad and hostile - really, the unideal witness, on the face of it - was none the less consistent. Unfortunately, most people in a typical jury will find her off-putting enough to be skeptical of her testimony, to one degree or another, upon hearing and seeing it in person. However, in the written court testimony transcript, which clarifies her accent and meaning, regardless of sketchy grammar, her testimony is compelling. When you then review her audio testimony again, you can see she is jaded and snide because she is annoyed by the parsing and repetition and even insulted by the constant demands for clarification. Her ethnic Haitian American Ebonics accent is really not that hard to understand, and she clarifies exactly who she heard when she quoted Martin saying, "Get off! Get off!" by repeating, "Trayvon!", with great emphasis, at the end of her original recorded witness interview just after the killing, the transcript of which was clearly incorrect as to the word, "could've" versus what she said, "could hear'(heard) Trayvon sayin' 'Get off Get off!' Trayvon!" when read along while listening to that recording.
Martin is dead, and this young woman is the only voice left for telling his side of it. The chances of that side of the story being buried under attorney focused doubts, and clueless, naïve jury derp, is what the defense is concentrating on. You could see the defense attorney's irritation when Jeantel sincerely and forcefully said of the transcript error, "Believe me when I tell you it's wrong...sir."