Would you shoot him then?
In the western movies I saw when I was young it was common for the hero to win a fist fight with the villain. Invariably the villain would then pull a gun and try to shoot the unarmed hero. I don't ever remember thinking that pulling a gun after he lost the fight was heroic or brave.
I'd like to think that one-on-one I would have beaten the tar out of the 17 year old boy, rather than the converse. I probably wouldn't try to be the neighborhood watch guy unless I could handle myself. If I was going to confront a young man who I thought was up to no good, (and I actually encountered this in my gated community earlier this week), I would try to plan the meeting so that I either stayed in my car or I would have the gun out and visible when I questioned his business in my community. But I also know that these situations can escalate, and things don't usually go according to plan.
I haven't convicted Zimmerman, or called him a murderer. But the idea of shooting an unarmed teenager on a public street still bothers me. And I certainly think an investigation of some kind is in order. If the local police have conducted such an investigation and found that Zimmerman acted with just cause, some sort of statement should be made to that effect.
I personally think the 2nd Amendment gives me the right to own an armed Apache helicopter, if I could afford one. In theory, I should be able to have the same weapons as the government to prevent tyranny. But this liberty to bear arms carries responsibility for my actions. If I hurt someone with my weapons, through accident, carelessness, poor judgment, etc., I must bear the consequences for my actions.
And I agree fully with your last statement. The police have already reviewed Zimmerman's actions and couldn't find anything to charge him with. That isn't good enough for some folks though...