It has been stated previously that Zimmerman shot Treyvon both because he was being attacked and because he believed that Treyvon saw the gun he had on his person. He then drew his gun and shot Treyvon. Treyvon Martin had injuries on his knuckles, showing that they were against something repeatedly (which matches up pretty well with him beating a man up), and the bullet wound. Zimmerman had a broken nose, bruising on the front and back of his head, and lots of scrapes and other things on the back of his head, which, while not concretely proving his head was pound against concrete, does most likely put him on the bottom, as well as the fact that his clothing was wetter on the back and had some grass stains (he did state he was trying to get to the grass to cushion some of the blows).
Now, it is entirely impossible for anyone to say who actually started the conflict. I don't believe Zimmerman should be charged with second degree murder, but I do feel he should be charged with manslaughter. As he had been taking some MMA lessons for some time, as well as the fact that if he was in said "ground and pound" position, he should have been able to put up at least some resistance at some point. I think that using his gun was an entirely uncalled for action, though, it was entirely for self defense. I would assume that he would at the least be able to put up his arms to block some of the hits, but, this is not the case (no reports of any defensive wounds on Zimmerman, only the head injuries). So, while the tapes can provide a bit of insight, we can't entirely trust them, or at the least, put them as circumstantial. Long story short, I think Zimmerman is guilty of overreacting, but it was a justified overreaction. Treyvon wasn't a saint (and was possibly paranoid, even possibly still under the effects of a drug at the time, and a possible long time drug user), but neither do I believe he should've been shot first.