That is the question, isn't it? Of course, mere words alone are not legal justification to use physical force against someone. The question is, who initiated the physical conflict?
Another thing that has come out is that Trayvon Martin was about 70-100 feet from his father's residence where the altarcation happened.
From the pictures and reports, he appears to be a fairly athletic 17 year old, and George Zimmerman does not appear to be in great shape.
One could ask, if Trayvon was afraid, why didn't he just run to his father's house?
Remember George Zimmerman had as much right to be in that public space as Trayvon Martin did.