So, just to clarify, not speaking to the specifics of the Trayvon Martin tragedy, but I can start a fight with someone (maybe verbally? maybe physically? a few insults and a few shoves) and when they punch me and I get scared I can pull out a gun and shoot them?

Yes, you can. Then you would be put against the jury of your peers and would have to convince them that you a) actually did not start the fight and b) reasonably had a reason to fear for your life. If you succeed at that (all while paying through the nose for a lawyer, unless you're a millionaire say bye-bye to your savings, and btw your job too - they won't keep your place while you're being arrested and judged) - you get to walk. Not my idea of a good time, though.