You're partially correct in that Trayvon Martin might have been in the wrong place at the wrong time, but note that George Zimmerman was a neighborhood watch captain in an area where there had been several burglaries / robberies in the several weeks preceeding the incident. Martin was not a familiar face to the area, so as a neighborhood watch captain Zimmerman took it upon himself to watch this kid. Everything Zimmerman did that evening was by the book in terms of neighborhood watch: he followed, he called 911, and he headed back to his truck when he was asked to do so by dispatch. It was when he was heading back to his truck that the incident occurred.
Martin was the only assailant here, and from the evidence presented Zimmerman did nothing to provoke the incident other than what he was allowed to do as part of neighborhood watch and Florida's Stand-Your-Ground law.