It isn't a chance encounter regardless of whether or not you are considering encounter to be broad or specific (I was assuming that you had not changed your usage).
In the broad sense, it is not a chance encounter because Zimmerman was patrolling the neighborhood, on the lookout for anyone he considered suspicious. He was actively seeking out people. Martin, a black teen wearing a hoodie and chatting on his phone, was apparently sufficiently suspicious. Yeah, in a really meaningless way it's a "chance" encounter. If I go deer hunting, and shoot a deer, it's by chance. No one should be surprised if I come back with one, though (excusing for the fact that I would never have the skill to successfully complete that hunt).
In the specific sense, referring to the fight, it is not a chance encounter because Zimmerman had already singled Martin out and, regardless of whether or not he began the fight, had certainly set into motion the events that would lead to one.
And, again, we are saying that we don't care if Martin started the fight. We don't care if Martin had the upper-hand in said fight when he was shot. We don't care if Martin went for Zimmerman's gun.
Our point is that Zimmerman had presented himself as a significant threat to Martin, regardless of whether or not he realized he was doing it, which voids his ability to claim self defense. The altercation between them, regardless of who threw the first punch, is directly related to Zimmerman's choice to follow Martin in his car, despite the request from the police that he not do that.
Had he listened to that dispatcher, Trayvon Martin would be alive today.
That's the point.
Anyways, you all are horrible, @#%^ed up people
Never underestimate the healing power of a massive dong.