The simple, uncontested fact that seems to be ignored is that an armed man pursued, confronted, shot, and killed an innocent, unarmed man. Zimmerman is known to have initiated the confrontation. He clearly said on the recorded phone call that he was pursuing Martin. It's one thing to defend oneself if one were simply attacked on the street or on one's own property, but in what kind of messed-up legal system would it be OK to follow somebody down a street at night for no demonstrable reason, chase him down, confront and shoot him when he fights back?
As for his story that he broke off the chase and Treyvon Martin attacked him afterward, that's a case that Zimmerman needs to prove because the known facts certainly don't support that version of events. He may be innocent until proven guilty, but when he claims "self defense" the burden of proof shifts onto him to support that claim.
The facts as we know them would clearly support a case of improper defense resulting in death: in other words voluntary manslaughter, or at least involuntary manslaughter if he could somehow prove that there was a struggle for the weapon.