I have no problem with him defending himself against a possible deadly assault (getting his head rammed into the concrete). The only thing I have a problem with is Zimmerman putting himself into that situation in the first place. While it wasn't illegal to follow Martin or even confront him as to his actions, it probably wasn't the smartest thing to do. But even then, there was and is no reason to throw him in jail. The only reason at all this ever went to trial was because of political pressure.
Christian apologetics is the art of rolling a dog turd in sugar and selling it as a donut.