We didn’t really hear about Trayvon until several weeks afterwards when the parents started raising the issue of the police refusing to prosecute Zimmerman. (From the evidence I have seen, I agree with the police, although there are some bits of information I would like to know, such as how the fight began in the first place.)
I don’t think police inaction will be an issue in this case, so the situations aren’t exactly parallel.
I don't believe that you'll ever know that to a metaphysical certainty, but my take, from Zimmerman's account (told by a combination of sources) ... he's lost Martin, so he walks away from his truck toward a street, to obtain an address. Maybe 50-100 foot walk. He turns around, and starts walking back to his truck. Martin had gone away at a 90 degree angle, down a walkway that is perpendicular to the one Zimmerman is on. After Martin passes this perpendicular walkway (running behind the buildings), Martin calls to him, from Zimmerman's left-rear. Zimmerman turns and faces Martin. Martin asks "You got a problem, homie?" Zimmerman says no, reaches for cellphone in his pocket, Marti says "You do now" and cold cocks Zimmerman.
Taking advantage of downing him, Martin sits on Zimmerman, and administers a head pounding against the walkway. Zimmerman calling out for help ...
Gun doesn't come out until it is revealed when Zimmerman shifts so the next head slam goes to the grass instead of the pavement. Martin sees the gun, and says something like "You're going to die tonight." Martin attempts to take the gun, which Zimmerman resists.
Martin could have walked home - the fight was 70 yards from the place he was staying, and he came from that direction in order to ask Zimmerman if Zimmerman had a problem.
No witness provides evidence to contradict any part of Zimmerman's account - not even the witnesses that later came out and gave "hostile" accounts. Those witnesses admit they didn't see the fight start, or go on, and only came to view after the shot was fired. A number of witnesses give equivocal testimony. That's typical. Not paying attention, couldn't see, too stressed to recall clearly, etc.
I assume the forensic evidence (powder burns, entry wound, injuries, skin under fingernails, blood) likewise does not contradict Zimmerman's account.