If anyone thinks I'm going to actually read through over 260+ responses, you're dreaming. So at the risk of my points having been already addressed before this, here goes.
1. If Mr. Martin had only good intentions in that neighborhood, and felt that he was being stalked by Mr. Zimmerman, why didn't Martin call the police? He had a cell phone. Normal, law-abiding people call the cops when they need help, they don't turn on their stalker, take the law into their own hands, and instigate a physical confrontation. Had Martin called 911, it is a certainty that the police would have called Zimmerman back and under those circumstances, we would have had a different result. Martin's failure to call the police means to me that he had something going on that he didn't want them to discover upon their arrival.
2. The time line works in Mr. Zimmerman's favor. Since a bullet to the heart tends to disable the recipient, Zimmerman's wounds - broken nose, etc., had to have been inflicted before Martin was shot and killed. That means Martin struck first.
3. When you call the police, you have no idea when they will actually arrive on the scene. There might be an officer a block and a minute away from you, or there might not be an officer available for a half hour. Some nights are busier than others. Some much less busy. The very last thing anybody contemplating a 2nd degree murder would do is call the police right before he attempts it. He might hear sirens and see lights even as he hangs up the phone. Never a good thing when contemplating a crime. Zimmerman knew he had been told to stay in his vehicle. He knew he was told the PD needed no further input from him. He knew the Neighborhood Watch rules and regulations. Knowing what he knew, it makes no sense whatsoever that he would have shot Martin unless he felt he absolutely had to. Knowing that he already exceeded his parameters by leaving his vehicle, and might already be in trouble with the PD and the HOA, he would have had to be insane to exceed them further by murdering Martin. Zimmerman is not insane. Somewhat noncompliant, but far from crazy. Regardless of how he got himself into that fracas with Martin, who also made his own share of bad decisions that night, there they were, perhaps equally where neither one should have been, and Zimmerman, injured, frightened out of his mind, in great pain, and in understandable fear of further injury, did what anyone with the option Zimmerman had at his disposable-specifically-use his weapon to defend himself.
4. Most events like this are inevitably found to be a "comedy of errors" - the result of bad calls and bad decisions all around. That fact alone mitigates to reasonable doubt.