If Trayvon was not up to no good, why would he confront Zimmerman physically.
If I was walking through a neighborhood and I noticed someone was following me, I'd turn and ask if there was something I could help that person with. By making the confrontation it would declare to that person that I am not a threat, and would start a dialog. If that person became aggressive, I would become defensive and I definitely know how to defend myself.
If I saw someone suspicious walking through my neighborhood I would shout out to them, from a reasonable distance, if there is something I could help them with. If they would run, 911 would get an instant call. If they would turn and start a conversation (benign) I would help them with whatever they needed, or even just make small talk till I felt they were not up to no good. If they turned and became aggressive see above.
If someone is looking for trouble they are sure to find it. If someone is just on the way to or returning from a local 7-11 why would they be looking for trouble. I guess I just think too logically for my own good.