Zimmerman got out of his SUV to follow Trayvon on foot.
That negates any self defense argument in my book.
Do nothing. Say nothing. Accept the consequences of letting criminals (etc) have free access to your community and possibly to your family or friends and neighbors.
A simple challenge of a suspicious character could possibly prevent an incident & “if” the person is innocent he would possibly appreciate the fact that there are concerned people who are watching and protecting the neighborhood and its residents.
I suggest you go drinking with a cop some time. I have a number of times, and they all say that "getting involved" is a very dangerous thing. Mr Zimmerman is done. He and his family will have to spend a long time in hiding, or leave the country. Was it worth it?
Now, there have been times where I have made a stand, but legal counsel has always been "Don't get involved". In this situation, getting out of the SUV will get Mr Zimmeran charged. It might a BS charge, but he started the contact.