If he wasn't breaking the law and I am off duty then I would call 911 and be a good witness. Let me also add that if I was concerned enough about the situation I would stick around to see it through when the police arrived. I have done so on such things as DWIs, & people getting out of control in a restaurant or store. But then again I am a cop and I carry concealed and have been trained how to respond. If he is committing a violent crime then I will gladly step in even off duty. I am obligated to do so in my profession and would proudly participate!
The facts remain to be seen whether or not Zimmerman physically confronted Martin or if it was just verbal and Martin attacked him when retreating as some have said.