allow me to preface this by saying this is all purely speculation, and that my guess (yes, its a guess) is as good as anybody's.
personally, if GZ's intention was to pick a fight or to murder the kid, i don't think he would call the cops to warn them first. if he had his vigilante fake cop complex, he'd probably serve "sweet justice" by quietly sneaking up on trayvon and putting him down quietly, not with a loud gunshot in plain sight
similarly, trayvon wasn't walking home with the intention of finding the first guy he saw and picking a fight with him. i'm not going to say he had vicious intent that night, but its not out of the realm of possibility that he was su ious of GZ following him and he reacted more aggressively than the situation called for. besides, isn't there some sort of confirmed timeline that shows martin had 4 minutes to get to his destination, which was 100 yards away at the time? by the end of the struggle, he lied dead 70 yards away, meaning he moved all of 30 yards in that timespan. if the struggle had gone on THAT long, for 3-4 minutes, surely there'd have been more eyewitnesses.
now if tomorrow some crazy groundbreaking evidence came out that proved Zimmerman was the aggressor/instigator i wouldn't be THAT shocked, or offended or anything stupid like that. i have no personal ties to GZ in any way shape or form. he's white/hispanic and i'm armenian which is more middle eastern than anything. but i don't see how any of the facts of the case outside of the damn race card makes GZ look like the instigator