I don't know about that. Putting myself into Zimmerman's shoes (armed, gun in waistband). As soon as I'm in a face-to-face confrontation with somebody and it 'might' get physical. I'm not thinking about throwing a punch or wrestling on the ground, because my number 1 concern is about the possibility of being disarmed.
Once Trayvon threw that punch he was, more likely than not, going to be looking at a drawn gun.
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Or, once Zimmerman showed the gun he was, more likely than not, going to get punched.
Point is...nobody knows which happened first.
Martin could have been scared to death of the stranger with the gun and fighting for his life. Only Zimmerman knows.